The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions

Abstract

The oblique extraocular muscles (EOMs) were dissected in 19 cetacean species and 10 non‐cetacean mammalian species. Both superior oblique (SO) and inferior oblique (IO) muscles in cetaceans are well developed in comparison to out‐groups and have unique anatomical features likely related to cetacean orbital configurations, swimming mechanics, and visual behaviors. Cetacean oblique muscles originate at skeletal locations typical for mammals: SO, from a common tendinous cone surrounding the optic nerve and from the medially adjacent bone surface at the orbital apex; IO, from the maxilla adjacent to lacrimal and frontal bones. However, because of the unusual orbital geometry in cetaceans, the paths and relations of SO and IO running toward their insertions onto the temporal ocular sclera are more elaborate than in humans and most other mammals. The proximal part of the SO extends from its origin at the apex along the dorsomedial aspect of the orbital contents to a strong fascial connection proximal to the preorbital process of the frontal bone, likely the cetacean homolog of the typical mammalian trochlea. However, the SO does not turn at this connection but continues onward, still a fleshy cylinder, until turning sharply as it passes through the external circular muscle (ECM) and parts of the palpebral belly of the superior rectus muscle. Upon departing this “functional trochlea” the SO forms a primary scleral insertion and multiple accessory insertions (AIs) onto adjacent EOM tendons and fascial structures. The primary SO scleral insertions are broad and muscular in most cetacean species examined, while in the mysticete minke whale (Balaenoptera acutorostrata) and fin whale (Balaenoptera physalus) the muscular SO bellies transition into broad fibrous tendons of insertion. The IO in cetaceans originates from an elongated fleshy attachment oriented laterally on the maxilla and continues laterally as a tubular belly before turning caudally at a sharp bend where it is constrained by the ECM and parts of the inferior rectus which form a functional trochlea as with the SO. The IO continues to a fleshy primary insertion on the temporal sclera but, as with SO, also has multiple AIs onto adjacent rectus tendons and connective tissue. The multiple IO insertions were particularly well developed in pygmy sperm whale (Kogia breviceps), minke whale and fin whale. AIs of both SO and IO muscles onto multiple structures as seen in cetaceans have been described in humans and domesticated mammals. The AIs of oblique EOMs seen in all these groups, as well as the unique “functional trochleae” of cetacean SO and IO seem likely to function in constraining the lines of action at the primary scleral insertions of the oblique muscles. The gimble‐like sling formed by SO and IO in cetaceans suggest that the “primary” actions of the cetacean oblique EOMs are not only to produce ocular counter‐rotations during up‐down pitch movements of the head during swimming but also to rotate the plane containing the functional origins of the rectus muscles during other gaze changes.

The superior and inferior oblique muscles in cetaceans have several unusual features including ‘functional trochleae’ formed by parts of adjacent rectus extraocular muscles (EOMs) and, in some species, elaborate scleral and accessory insertions (AIs). The trochleae and AIs are significant connections between the oblique and rectus EOMs and likely both constrain oblique actions at their primary scleral insertions and allow the obliques to directly influence actions of the recti.

1. INTRODUCTION

The roles of eye movements in cetacean visual behaviors are largely unknown. Informal behavioral observations suggest that eye movements that stabilize the visual field on the retina during self‐motion are well developed in many cetaceans. These behaviors include the excellent visuomotor coordination seen in captive and wild dolphins (Mobley & Helweg, 1990), the visual targeting of prey by spy‐hopping in near‐shore or ice‐floe environments seen with killer whales (Pitman & Durban, 2011) and the apparently vivid visual attention paid to conspecifics, large fishes and human divers by both odontocetes and mysticetes in well‐lit surface waters as seen in numerous video documentaries. While visuomotor behaviors are hardly known for many cetacean species, it is nevertheless likely that robust three‐dimensional compensatory eye movements during locomotion are present in most. The ocular movements involved in all these behaviors likely include significant “torsional” components where the globe is rotated about the visual axis (cycloductions). The primary muscle actions in vertebrate torsional eye movements are generally provided by the oblique extraocular muscles (EOMs), namely the superior (SO) or dorsal oblique and the inferior (IO) or ventral oblique (Fink, 1962; Prince, 1956; Spencer & Porter, 1988; Walls, 1942). The gross anatomy of cetacean oblique EOMs and especially a complex series of accessory insertions (AIs) and fascial connections found in our dissections are examined in this study. A detailed review of the primary and accessory (sometimes called secondary) insertions of human oblique EOMs is provided to give context to these largely unfamiliar anatomical structures.

1.1. The oblique EOMs in vertebrates generally

The anatomy, action, and innervation of oblique EOMs are among the most highly conserved of vertebrate characteristics as a pair of oblique muscles are found in lampreys and all gnathostome taxa (Suzuki et al., 2016; Young, 2008). The homology of the lamprey anterior and posterior obliques and the gnathostome IO and SO, respectively, are supported by comparative embryology (Neal, 1918), anatomy (Nishi, 1938), neuroanatomy (Fritzsch et al., 1990), paleontology (Young, 2008), and developmental biology (Suzuki et al., 2016). The function of the oblique EOMs is likewise highly conserved despite differences in orbit geometries and head movement patterns among vertebrates. In general, the oblique EOMs mediate torsional eye movements around the line of sight (Bianco et al., 2012; Fink, 1962; Wall, 1942). The directional pull of oblique EOMs depends on the orientation of the optical axis relative to the head, with the SO pulling the top of the eye rostrally in lateral‐eyed species and medially in frontal‐eyed ones like humans. Likewise, the IO pulls the bottom of the eye rostrally in lateral‐eyed and medially in frontal‐eyed species. The oblique EOMs can also participate in vertical and horizontal eye movements depending on the precise orbital geometry of different species (Cox & Jeffery, 2008; Ezure & Graf, 1984). Vertebrate oblique EOMs generally insert at or in front of the equator of the globe (Duke‐Elder, 1958; Prince, 1956; Wall, 1942). The insertions of SO and IO onto the posterior hemisphere of the globe as seen in humans and some other primates allows them an increased role in vertical eye movements compared to their more general functions of driving cycloductions (Fink, 1962). The precise roles of oblique EOMs in non‐torsional eye movements (i.e., vertical and horizontal) are not well‐studied outside of primates.

1.2. Oblique EOM structure and function in humans: Pulleys, compartments, and AIs in human orbital mechanics

The best‐characterized oblique EOMs are of course those of humans and other frontal‐eyed primates in which the orbital neuromuscular anatomy (Demer et al., 2003; Kono et al., 2005; Le et al., 2015; Spencer & Porter, 1988), central neural circuitry (Büttner‐Ennever, 2007; Leigh & Zee, 2016; Spencer & Porter, 2006), and role of oblique muscles in eye movements (Demer, 2007, 2017) are well established. Clinically and functionally the human obliques are considered members of the “vertical eye muscles” since they complement the superior (SR) and inferior (IR) rectus muscles in elevating and depressing gaze (Clark & Demer, 2016; Fink, 1962; Leigh & Zee, 2016). Acting more in isolation the oblique muscles mediate torsional eye movements in response to lateral head tilts with SO driving intorsion (incyclotorsion =incycloduction) and IO, extorsion (excyclotorsion =excycloduction) (Demer, 2017; Leigh & Zee, 2016). The latter roles are especially noticeable clinically in paralysis of SO and IO with resulting compensatory head tilt (Fink, 1962; Leigh & Zee, 2016; Wu & Yan, 2020).

Current understanding of human EOM structure and function is based on insights gained from magnetic resonance imaging (MRI) studies of orbital morphology during normal and pathological eye movements (Demer et al., 1995; Miller, 1989; reviewed in Demer, 2007, 2017) combined with decades of histoanatomical, neuroanatomical and neurophysiological studies of oculomotor function (Kono et al., 2002; Leigh & Zee, 2016; Spencer & Porter, 1988, 2006). Together they yield a picture of compartmentalized EOMs able to modulate their own and each other's pulling directions through insertions not only onto the globe but also onto surrounding fibromuscular fascia. The MRI studies led to discovery that the suspension of EOMs in the orbital fascia forms a system of mobile connective tissue pulleys that serve as the functional mechanical origins of the EOMs (Demer, 2007; Demer et al., 1995; Miller, 1989). The pulleys are sleeves of connective tissue that surround each EOM as it penetrates the Tenon's fascia. The histological stratification of mammalian EOMs into global and orbital layers (Spencer & Porter, 1988) is an integral part of the function of the EOM pulley system (Kono et al., 2002). The EOM global layers contain a large percentage of glycolytic twitch fibers suitable for phasic activity and insert into the sclera to generate oculorotory forces. In contrast, the orbital layers, which contain a high percentage of fatigue‐resistant oxidative twitch fibers more suitable for continuous tonic activity, control the pulling directions of the EOMs by inserting onto their connective tissue pulleys (Demer, 2017; Spencer & Porter, 2006). Further compartmentalization of the global layers and their scleral insertions allows additional fine‐tuning of EOM pulling directions by differential contraction within individual EOMs (Demer, 2015; and see Discussion below). Finally, the scleral and orbital insertions of oblique EOMs in humans appear to vary much more than those of the rectus EOMs, with consequences for clinical and surgical practice (De Angelis et al., 1999; Demer, 2017; Fink, 1962; Yalçin & Ozan, 2005).

The human SO originates from the medial bony wall at the apex of the orbit just above the common tendinous ring (Dutton, 2011). It extends anteriorly as a fleshy cylinder running along the medial wall of the orbit superior to the MR and becomes tendinous before reaching the cartilaginous trochlea located on the superior nasal part of the frontal bone. As with other EOMs there are two distinct layers within the SO, a deeper global layer surrounded by an orbital layer, each with different muscle fiber types, innervation patterns and functional properties (Büttner‐Ennever, 2007; Kono et al., 2005; Spencer & Porter, 1988, 2006). The global layer of the human SO passes through the trochlea and turns posterolaterally before passing inferior to the tendon of the SR to insert posterior to the equator of the globe on its superolateral quadrant, a position well‐suited for vertical as well as torsional roles. The orbital layer of SO continues as the SO “sheath” which departs the trochlea to attach to the connective tissue pulley of the SR (Kono et al., 2005). The trochlear nerve (CN IV) enters the orbital surface of SO proximally near the origin of the muscle and solely supplies this muscle. The actions of SO are intorsion, depression and abduction of the eye. These actions are dependent on the direction of gaze with intorsion dominating when the eye is abducted and depression dominating when the eye is adducted. Since the torsional and horizontal actions of SO are opposite in direction to those of the IR, the joint action of SO and IR yields nearly pure depression of gaze (infraduction).

Unlike the SO and rectus EOMs, the human IO originates from the orbital surface of the superior maxilla a few millimeters behind the lower orbital margin. The IO runs laterally and posteriorly to circle the ventral globe, staying inferior to the IR, and then the central IO fibers (global layer) insert, without much or any length of tendon, onto the sclera between the insertions of IR and LR on the inferolateral quadrant of the posterior hemisphere of the globe. The orbital layer of IO inserts onto the pulleys of the adjacent IR and LR muscles (Demer et al., 2003). Branches from the inferior ramus of the oculomotor nerve (CN III) enter the muscle near where the IO and IR cross. The classical actions of the IO are extorsion, elevation and abduction, with extorsion dominating when the eye is abducted and elevation dominating when the eye is adducted. The torsional and horizontal actions of IO are opposite in direction to those of the SR and thus together they mediate elevation of gaze (supraduction).

We refer to the “classical” actions of oblique EOMs to distinguish them from the additional roles discovered much more recently through MRI imaging of EOMs during eye movements (Demer, 2007; Demer et al., 1995), namely their role in driving an outer suspensory gimbal that rotates the inner suspensory gimbal comprising the pulley system of the rectus EOMs (Demer, 2019). This function is mediated by the muscular and fascial connections between the pulleys and muscle sheaths of the oblique EOMs and the pulleys of the rectus EOMs.

1.3. Oblique muscles in cetaceans

The general anatomy of the orbit and EOMs in cetaceans was described in our previous paper on circular orbital muscles (Meshida et al., 2020) and following a summary of those results the introduction will highlight cetacean oblique muscles described in the literature. Briefly, cetaceans have “open” orbits with the only bony wall being the orbital roof formed by the curved orbital plate of the frontal bone (Figure 1a–d; Mead & Fordyce, 2009; Meshida et al., 2020). The orbital contents are contained in a cone shaped sack of periorbita lined in part by a well‐developed sheet of circular muscle which we refer to as the external circular muscle (ECM; Figure 1b). The cone of ECM surrounds the SO and rectus EOMs, which in turn surround an internal circular muscle layer (ICM) and, deep to that, a substantial cone‐shaped retractor bulbi which covers an ophthalmic vascular rete mirabile which surrounds the optic nerve (see Meshida et al., 2020). The four rectus EOMs in cetaceans (superior/dorsal, inferior/ventral, medial/rostral, and lateral/caudal recti) are extraordinary in having very large outer palpebral bellies that insert into the eyelid structures and smaller inner scleral bellies that insert with thin, flat tendons onto the equator of the ocular globe (Video S1). The SO muscle originates at the apex of the orbit along with the rectus EOMs and retractor bulbi (RB) and the IO from the maxilla toward the rostral margin of the orbit, as is typical for most mammals. Because of the great depth of cetacean orbits, the recti, and obliques are exceptionally long compared to other mammals and are fleshy throughout their lengths (Zhu et al., 2000). The paths of the SO and IO leading to their scleral insertions are unique compared to other mammals, requiring them to traverse the palpebral bellies of rectus muscles and also involve sharp changes in direction at what we are calling “functional trochleae.” In many of the species we examined, both obliques also have complicated AIs joining them to other EOMs and connective tissue structures (see Results).

FIGURE 1.

The course of superior oblique (SO) and inferior oblique (IO) in cetaceans. (a) orbital and oblique extraocular muscle (EOM) landmarks are drawn on a ventral view of the left bony orbit in a bottlenose dolphin (Tursiops truncatus), showing the bony origins of SO (red) and IO (blue) as well as the preorbital process (PP) and the spot where the anatomical trochlea is fixed to the frontal bone (asterisk). Dashed lines indicate antorbital and postorbital ridges which provide attachment for the external circular muscle (ECM)/periorbita. (b) Proximal course of IO seen in ventral view in situ in another bottlenose dolphin (T. truncatus, USNM 594664, L). The IO adheres to the bony orbit until nearly the place (blue asterisk) where it penetrates the ECM and the palpebral belly of IR. The tendinous cone (TC) that serves as common origin of SO and recti is partially visible through dissected branches of maxillary artery (MA). (c) Proximal course of SO in dorsal view in the same specimen as (b) but with orbital contents reflected. The SO attaches to the cartilaginous trochlea by a fibrous sling (asterisk), cut here, runs toward the eyelid, parallel to the IO, and then turns caudally to penetrate the ECM and palpebral belly of superior rectus (SR). The locations of the pterygopalatine fossa (PPF) and ventral infraorbital foramen (IOF) are shown along with the maxillary nerve (MN) and MA and trochlear nerve (CN IV). (d) Lateral view of the left bony orbit of a bottlenose dolphin showing frontal (F), maxillary (M), lacrimal (L), and jugal bones (J) and the attachment sites of the SO, IO, and anatomical trochlea (asterisk). (e) Schematic diagram of a lateral view of a generic cetacean globe showing the distal paths and insertions of the SO and IO relative to the scleral insertions and palpebral bellies of the four rectus EOMs. The SO is shown passing internal (=medial =posterior) to the scleral tendon of SR while the IO passes external (=lateral =anterior) to the scleral tendon of IR. (f) Relations of the oblique muscles to the scleral bellies of rectus EOMs are schematically diagrammed on a mid‐orbital plane based on the MRI of Risso's dolphin seen in Figure 2. Lateral structures on the globe are also depicted (retractor bulbi [RB], LR, sclera, SO, and IO insertions (dotted). The dorsal and ventral parts of the schematic show relationships at the red arrows in Figure 2g,i. The SO runs posterior (toward apex of the orbit) to the scleral and palpebral belly of SR while the IO runs between the palpebral and scleral bellies of IR. The relation of the SO to the SR, and the IO to the IR applies to all cetacean specimens examined. Scale bars: 10 mm

Hunter (1787) provided a succinct description of the obliques in a few species of cetaceans and also a functional statement that was superior to those of the next two centuries: “The two oblique muscles are very long; they pass through the muscles of the eyelids, are continued on to the globe of the eye, between the two sets of straight muscles, and at their insertions are very broad; a circumstance which gives great variation to the motion of the eye.” A century later Weber (1886) provided detailed descriptions of the SO and IO in the northern bottlenose whale (Hyperoodon ampullatus), correctly determining the muscle origins, paths through the palpebral rectus muscles, sharp turns at trochlea‐like structures and multiple insertions not only into the sclera but also along the surface of RB and adjacent structures. Unfortunately, he was under the impression that whale eyes were immobile, so functionally his view was a regression from Hunter's. Other descriptions by Blainville (1822) and Schulte (1916) were consistent with Weber but nowhere near as detailed. The only modern detailed description of cetacean oblique muscles is that of Zhu et al. (2000) who used dissection and MRI to provide a very complete account of the eye muscles of the bowhead whale, Balaena mysticetus. They described the redirection of the SO course in detail as follows: after going toward the eyelid, the SO changes its direction gradually 90° caudally after passing a “tunnel‐like arrangement of connective tissue” instead of a cartilaginous trochlea. The SO then changes its direction another 90° toward the scleral insertion. The second redirection of the SO is caused by the continuation of the fibrous connective tissue tunnel associated with the dorsal rectus (SR) and an adjacent muscle possibly homologous with the levator palpebrae superioris (LPS). They described a similar change in direction of the IO along its course and attributed a likely significant role in eye movements to these muscles.

Among these previous studies only Weber (1886) described AIs of the cetacean oblique muscles, noting a split insertion tendon of the IO and spread of insertion fibers onto the RB. Motais (1887) gave detailed accounts of complex insertions and interconnections between oblique and rectus tendons in horse and cow, along with comparisons to the anatomy of human EOMs and Tenon's fascia. As with Weber's studies on cetaceans, the monograph by Motais remains a valuable primary source. Our dissections revealed considerable variation in these AIs within cetaceans and prompted study of non‐cetacean species as well to check for generality of these features. The present study will describe the detailed anatomy of the superior and inferior oblique muscles with focus on characteristics that are present uniquely in cetaceans and will discuss their possible functions including those of (1) their fleshy and broad scleral insertions in contrast to those of rectus EOMs, (2) the peculiar “trochlea‐like” structures that constrain and redirect these muscles as they course through the other layers of orbital muscles, and (3) the intricate and complex AIs of SO and IO.

This paper is the second in a series of three on cetacean eye muscles. The first (Meshida et al., 2020) covered circular muscles of the orbit and the third paper, forthcoming, will focus on the cetacean rectus EOMs with their extraordinary palpebral portions. The overall muscular functional anatomy of cetacean eyes will also be evaluated in that paper. The present paper contains many images and facts about parts of the rectus EOMs but is only concerned with them as related directly to the oblique EOMs. Likewise, while the SO and IO are undoubtedly involved in multiple kinds and directions of eye movements, the functional focus of the present paper is the most obvious of those, cycloductions that occur during typical locomotory head pitching movements.

2. MATERIALS AND METHODS

2.1. Dissection

The orbits of 31 cetacean specimens (40 orbital specimens) and two non‐cetacean specimens were collected at the Osteopreparation Laboratory of the Smithsonian Museum Support Center in Suitland (MD) and the Virginia Aquarium and Marine Science Center Foundation (VA) (Table 1). In addition, one sirenian and nine non‐cetacean orbital specimens were collected from Florida Wildlife Conservation Commission (FL), the Smithsonian National Zoological Park (DC), and the Mercer County Wildlife Center (NJ) for out‐group comparison. All the subjects perished by natural causes or were humanely euthanized by authorized organizations; no animals were acquired or sacrificed for this study. Permission to use the materials, in some cases for destructive analysis, was granted by each of the institutions listed above and in Table 1. The globe, eye muscles and surrounding tissues were extracted and preserved in 70% ethanol after fixation in 10% formalin, except the pygmy hippopotamus (Hexaprotodon liberiensis, USNM 256491): this specimen was already preserved, and the orbits were dissected in situ. The orbits were dissected macro‐ and microscopically and were photographed and videoed with Sony HDR‐CX405, Dino‐Lite AM4115ZT, Nikon D3400 and D610 digital cameras. Images and videos were edited in Adobe Photoshop and Premiere, and figure plates were assembled in Inkscape. Videos [Link], [Link], [Link] showing overviews and details of EOMs in minke whale and pygmy sperm whale are included in Supporting Information.

TABLE 1.

Species examined in this study

Catalogue #	sub#	Group	Family	Species	Common name	Side
USNM 504674	1,2	Mysticeti	Balaenopteridae	Balaenoptera acutorostrata	Minke whale	L, R
USNM 593554	3	Mysticeti	Balaenopteridae	Balaenoptera acutorostrata	Minke whale	R
USNM 594656	4	Mysticeti	Balaenopteridae	Balaenoptera acutorostrata	Minke whale	L
USNM 594182		Mysticeti	Balaenopteridae	Balaenoptera physalus	Fin whale	R
USNM 594183		Odontoceti	Physeteridae	Physeter macrocephalus	Sperm whale	R
USNM 572142	1.2	Odontoceti	Kogiidae	Kogia breviceps	Pygmy sperm whale	L, R
USNM 593969	3	Odontoceti	Kogiidae	Kogia breviceps	Pygmy sperm whale	R
USNM 594027	4.5	Odontoceti	Kogiidae	Kogia breviceps	Pygmy sperm whale	L, R
USNM 571927	1.2	Odontoceti	Ziphiidae	Mesoplodon densirostris	Blainville's beaked whale	L, R
USNM 550070	1.2	Odontoceti	Ziphiidae	Mesoplodon europaeus	Gerveis’ beaked whale	L, R
USNM 550825	3	Odontoceti	Ziphiidae	Mesoplodon europaeus	Gerveis’ beaked whale	R
USNM 594566	4	Odontoceti	Ziphiidae	Mesoplodon europaeus	Gerveis’ beaked whale	R
USNM uncatalogued		Odontoceti	Ziphiidae	Mesoplodon sp.	beaked whale sp	R
USNM 594065		Odontoceti	Monodontidae	Delphinapterus leucas	Beluga whale	L
USNM 594045		Odontoceti	Delphinidae	Globicephala macrorhynchus	Short‐finned pilot whale	R
USNM 594001		Odontoceti	Delphinidae	Grampus griseus	Risso's dolphin	R, L (MRI)
USNM 594200	1.2	Odontoceti	Delphinidae	Lagenodelphis hosei	Fraser's dolphin	L, R
USNM 571446		Odontoceti	Delphinidae	Lagenoryhnchus acutus	Atrantic white‐sided dolphin	R
USNM 550008		Odontoceti	Delphinidae	Peponocephala electra	Melon‐headed whale	R
USNM 504419		Odontoceti	Delphinidae	Stenella coeruleoalba	Striped dolphin	R
USNM 594532	1.2	Odontoceti	Delphinidae	Stenella frontalis	Striped dolphin	L, R
USNM 594181		Odontoceti	Delphinidae	Stenella longirostris	Spinner dolphin	R
USNM 594052		Odontoceti	Delphinidae	Steno bredanensis	Rough‐toothed dolphin	R
USNM 571618	1	Odontoceti	Delphinidae	Tursiops truncatus	Bottlenose dolphin	R
USNM 594054	2	Odontoceti	Delphinidae	Tursiops truncatus	Bottlenose dolphin	R
USNM 594531	3	Odontoceti	Delphinidae	Tursiops truncatus	Bottlenose dolphin	R
USNM 594664	4	Odontoceti	Delphinidae	Tursiops truncatus	Bottlenose dolphin	L (in situ)
USNM 593411	1	Odontoceti	Phocoenidae	Phocoena phocoena	Harbor porpoise	R
USNM 593413	2.3	Odontoceti	Phocoenidae	Phocoena phocoena	Harbor porpoise	L, R
USNM 593568	4	Odontoceti	Phocoenidae	Phocoena phocoena	Harbor porpoise	R
USNM 594066		Odontoceti	Phocoenidae	Phocoenoides dalli	Dall's porpoise	R
USNM 256491		Cetartiodactyla	Hippopotamidae	Hexaprotodon liberiensis	Pygmy hippopotamus	R
USNM 602250		Carnivora	Felidae	Puma concolor	Florida panther	L
NZP 114768		Carnivora	Mustelidae	Amblonyx cinereus	Asian small clawed otter	R
KM001		Cetartiodactyla	Cervidae	Odocoileus virginianus	White‐tailed deer	R
KM016		Cetartiodactyla	Bovidae	Capra aegagrus hircus	Goat	R
KM005		Carnivora	Procyonidae	Procyon lotor	Raccoon	R
KM020		Carnivora	Mephitidae	Mephitis mephitis	Skunk	R
KM003		Rodentia	Sciuridae	Marmota monax	Woodchuck	R
KM009		Rodentia	Sciuridae	Sciurus carolinensis	Gray squirrel	R
KM010		Rodentia	Sciuridae	Sciurus carolinensis	Gray squirrel	L
KM018		Lagomorpha	Leporidae	Sylvilagus floridanus	Cottontail rabbit	R
MEC16111		Sirenia	Trichechidae	Trichechus manatus	Florida manatee	R

Sub #s are assigned for the species with multiple specimens in numerical and alphabetical order.

Abbreviations: KM, specimens from Mercer County Wildlife Center, NJ; MEC, Florida Fish and Wildlife Commission; NZP, National Zoological Park; USNM, National Museum of Natural History.

2.2. Magnetic resonance imaging

MRI series of orbital contents of a Risso's dolphin (Grampus griseus, USNM 594001 L), a melon‐headed whale (Peponocephala electra, USNM 550008 R), and a sperm whale (USNM 594183 L) were acquired with a Bruker AVANCE III 7 T research MRI (Bruker BioSpin) using a 90 mm quadrature volume coil (RAPID Biomedical) at the Molecular Imaging Laboratory, Department of Radiology, Howard University College of Medicine, Washington, DC. In addition, orbital contents of a minke whale (Balaenoptera acutorostrata, USNM 593554 L,R) and a sperm whale (Physeter macrocephalus, USNM 594183 R) were scanned with a GE Signa HDxt 3 T clinical MRI (General Electric Medical Systems) with a quadrature knee/ft coil (GE) at the Department of Radiology in the Howard University Hospital, Washington, DC. Scans were acquired using T1‐ and T2‐weighted 2D fast spin echo (RARE) sequences with fat suppression. T1‐weighted scans were acquired with TE = 7 ms, TR = 1750 ms, NA = 16, RARE factor = 4, 45 × 2 mm slices, FOV = 90 × 80 mm, matrix = 256 × 512 pixels. T2‐weighted scans were acquired with TE = 33 ms, TR = 5000 ms, NA = 16, RARE factor = 8, 45 × 2 mm slices, FOV = 90 × 80 mm, matrix = 256 × 512 pixels. MRI data sets were analyzed and manually segmented in ImageJ (NIH). Video S4 shows unlabeled MRI slices of the Risso's dolphin imaging series side‐by‐side with EOM structures segmented in colors.

2.3. Abbreviations

A, apical, toward orbital apex; AI, accessory insertion; c, conjunctiva; cg, conjunctival gland; CN II, optic nerve; CN III, oculomotor nerve; CN IV, trochlear nerve; CN VII, facial nerve; CT, connective tissue; D, dorsal; ECM, external circular muscle (orbitalis); EOM, extraocular muscles; F, frontal bone; FOV, field of view; FT, functional trochlea; FTi, FTs, functional trochlea of IO, SO; gl, global layer; h, Harder’s gland; ICM, internal circular muscle; IO, inferior oblique; IOF, infraorbital foramen (ventral); IR, inferior rectus; J, jugal (zygomatic) bone; L, in figures, lacrimal bone; L, in text, left; LPS, levator palpebrae superioris; LR, lateral rectus; M, maxilla; MA, maxillary artery; MN, maxillary nerve; MR, medial rectus; MRI, magnetic resonance imaging; N, nasal; n, nerve; NA, number of averages; p, palpebral belly; PP, prefrontal process; PPF, pterygopalatine fossa; orb, orbital layer; ORM, ophthalmic rete mirabile; R, right; RARE, rapid acquisition with refocused echoes; RB, retractor bulbi; s, scleral belly; SO, superior oblique; SR, superior rectus; t, trochlea; T, temporal; TC, tendinous cone; TE, echo time; TR, repetition time; V, ventral.

3. RESULTS

3.1. The course of the oblique muscles in cetaceans

Unlike in humans, following the course of cetacean oblique muscles from origin to insertion is challenging and only a few accounts from dissections and modern imaging are available (Weber, 1886; Zhu et al., 2000). The difficulties arise from the size, rarity, and location of specimens along with the need to limit destructive methods with most museum specimens. The main difficulty is to establish the exact origins of the SO and IO. This has not been possible in most of our orbital specimens since it requires an intact orbit to be either scanned by MRI or dissected in situ. Either method requires a whole or half head that fits in a scanner or and/or can be destructively dissected. We have only accomplished this fully in a specimen of bottlenose dolphin.

3.1.1. In toothed whales

The oblique EOMs in the bottlenose dolphin (Tursiops truncatus) are very muscular and cylindrical along their entire courses. The SO originates from two sites. First, a strong tendinous cone that extends outwards from the bony cone‐shaped borders of the optic canal, similar to the common tendinous annulus in humans but extending much farther away from the apex (Figure 1a–d). Like the annulus, it serves as an origin for all the EOMs except the IO. The second origin of the SO is a strong fibrous attachment to the frontal bone a few millimeters dorsomedial to the tendinous cone. The SO courses away from the apex medial to the SR and superomedial to the MR and is surrounded by a connective tissue sheath that becomes more evident as the muscle proceeds peripherally. As it approaches the globe, the SO gives off a strong fibrous attachment to a cartilage fused to the antorbital ridge of the frontal bone (Figure 1a,c,d). By location and appearance this cartilage seems to be homologous to the typical mammalian trochlea, but there is much less change in direction of the SO at this point in the dolphin. The trochlear nerve (CN IV), which entered the orbit by the superior orbital fissure, runs along the external surface of the SO, to penetrate the muscle at the level of the trochlea (Figure 1c). After passing the trochlea, the SO then changes its direction about 120°, first toward the eyelid and then caudally to penetrate the ECM and palpebral belly of SR. As the SO enters the ECM and SR its sheath is a well‐developed fibrous connective tissue tunic. The passage through the ECM and SR during the sharp turn of the SO forms, in effect, a “functional trochlea,” described below in detail. The SO emerges from the palpebral belly of SR just medial to the scleral belly of SR, then runs posterior to both these bellies of SR and anterior (distal) to the RB to insert onto sclera of the anterior hemisphere of the globe near the equator (Figure 1e,f). In contrast to the tendinous scleral insertions of the rectus EOMs, the SO insertion is broad, and fleshy. Although there is a small connection to the connective tissue between SR and RB (CT, Table 2), the SO in the bottlenose dolphin does not otherwise have insertions other than onto the sclera.

TABLE 2.

SO insertion patterns in cetaceans and non‐cetaceans

The IO in bottlenose dolphin originates from the maxilla posterior and lateral to the area of the pterygopalatine fossa and the ventral infraorbital foramen (Figure 1a,b). The most medial part of the IO origin was in the form of three tendinous strips that fanned out on the maxilla. The rest of the substantial area of origin comprised a firm attachment of the dorsal side of the IO to the periosteum. The IO muscle belly runs laterally toward the eyelid, changes its direction about 120°, and pierces the ECM and the palpebral belly of IR (Figures 1b,c and 6d). At the point of changing its direction, the IO is surrounded by a fibrous sheath that has attachments to surrounding connective tissue. The IO, thus, has a functional trochlea like that of SO (below). After passing through the palpebral belly of IR, the IO continues between the scleral and palpebral IR bellies (Figures 1e,f and 6d for exposed view). It then emerges from between the IR layers and inserts onto the sclera between the scleral insertions of IR and LR, immediately distal to the RB insertion (Figure 1e).

FIGURE 6.

The course of inferior oblique (IO) (blue dotted line) in toothed and baleen whales is shown in ventral internal views. (a,b) IO pierces the ECM and IR to enter the functional trochlea of IO in the Dall's porpoise (Phocoenoides dalli, USNM 594066 R). The ECM is reflected in (b). (c) The IO is held by the well‐developed functional trochlea (*) and then runs between the scleral (s) and palpebral (p) bellies of IR in the Risso's dolphin (Grampus griseus, USNM 594001 R) The IR was split artificially into medial (*) and lateral portions during the dissection. (d,e) The scleral belly of IR was severed (yellow dotted lines) to show the IO coursing between the two bellies of IR after passing the functional trochlea of IO in the bottlenose dolphin (Tursiops truncatus, USNM 594561 R) and the Fraser's dolphin (Lagenodelphis hosei, USNM 594200 R). The accessory insertion onto the palpebral belly of IR (red dashed line) is also shown in the bottlenose dolphin. (f) IO is held by the functional trochlea (partially missing in this preparation) before running between the two bellies of IR in the minke whale (Balaenoptera acutorostrata, USNM 593554 R). Blue dotted lines, course of IO; yellow dotted lines, scleral belly of IR (cut edges); red dotted lines, accessory insertion onto the palpebral belly of IR. Scale bar: 10 mm

The inferior ramus of the oculomotor nerve (CN III) penetrates the palpebral belly of IR and comes out of the external surface running obliquely toward the medial margin of IR. CN III then dives between the scleral and palpebral bellies of IR, gives off multiple branches, and supplies both IO and IR.

The courses of the SO and IO described here are demonstrable in another odontocete in an MRI series of the left orbital contents of a Risso's dolphin (Figure 2; Video S4). The section plane is roughly orthogonal to the central orbital axis (apex to pupil center) and the sequence of selected sections (a–i) runs from deeper in the orbit (a) to shallower (i). The deepest slice (a) lies just deep to the functional trochlea of the SO. The SO and IO are positioned near each other at the rostral/medial edge of the orbit and the rectus EOMs are arrayed around the RB and the deeper central optic neurovascular core (Figure 2d, ORM). In slices b–e, the SO passes through the medial part of SR which binds the SO and serves as the major component of the functional trochlea of SO (FTs). The SO stays posterior (toward the apex of the orbit) to both the scleral and palpebral bellies of SR throughout its course (Figure 2b–g). Similarly, but in a more complicated manner, the IO penetrates the medial part of the IR, which holds IO and serves as its functional trochlea (FTi) (Figure 2g–i). A critical difference between SO and IO paths is that the IO runs between the scleral and palpebral portion of IO and thus passes anterior to (=ventral to, outside of) the scleral tendon of IR (Figure 2h,i). The separation between scleral and palpebral bellies of the MR, SR and LR are more clearly seen than that of IR. The bright ring of tissue running between the scleral and palpebral bellies of the recti is the conjunctival gland tissue which is directly in contact with the IO sheath ventrally (Figure 2g–i).

FIGURE 2.

Images from a T1 RARE MRI series of the orbital contents of a Risso's dolphin (Grampus griseus—USNM 594001 L) demonstrate the course of superior oblique (SO) and inferior oblique (IO) through their respective “functional trochleae”. The image plane is roughly orthogonal to the orbital axis and runs from more apical in the orbit (a) to more distal, toward the outer surface of the eye (i). (a,b) The serial images show that SO and IO come close together before turning to pass toward their insertions. (b–d) The medial division of superior rectus (SR) wraps and holds SO serving as the functional trochlea of SO (FTs). (d–g) SO runs posterior to both bellies of SR throughout its course. (f–i) IO traverses inferior rectus (IR) palpebral belly which holds IO and serves as the functional trochlea of IO (FTi) and then runs between IRp and the scleral tendon of IR. The red arrows in g (dorsal) and i (ventral) correlate to the schematic in Figure 1f. Color shading: red: SO, dark blue: IO, magenta: SR, yellow: medial part of IR, tan: IR, green: MR, light blue: LR, bronze: RB. Scale bar: 10 mm. Numbers in lower right are from the full 43 slice MR series. Please see additional slices of this series with both painted and original slices side‐by‐side in Video S3

3.1.2. In baleen whales

The general features of the oblique EOMs in baleen whales are similar to those in toothed whales with differences mainly related to the size of orbital contents due to the typically larger, wider skulls of baleen whales. We have not been able to follow the obliques all the way to their origins in any of our baleen whale specimens since both SO and IO had to be severed to remove the orbital contents and the orbital apex was too deep for further dissection. Schulte’s (1916) description of a fetal fin whale (Balaenoptera physalus) and the MRI data of Zhu et al. (2000) in bowhead whale (Balaena mysticetus) show that the SO originates from a common tendinous structure around the optic nerve along with the rectus EOMs. The similarities between oblique EOMs in baleen and toothed whales are numerous and include the origins of SO and IO, the presence of a small anatomical SO trochlea, both muscles being roughly cylindrical and surrounded by fibrous connective tissue sheaths, the passage through rectus and circular muscles forming FT of both SO and IO, locations of primary insertions on the sclera, presence of AIs and innervation pattern. The main difference between mysticetes and odontocetes is that instead of forming fleshy insertions as in odontocetes (e.g., Figure 3a–c), the SO and IO in mysticetes have broad fibrous (aponeurotic) scleral insertions as in the SO of fin whale and minke whale (Figure 3d–f) which fan out to form very broad tendinous scleral insertions after passing through the ECM and palpebral belly of the SR. As in bottlenose dolphin, the IO in the baleen whales exited the palpebral belly of the IR anterior to the scleral belly of the IR before inserting to the sclera. Hence, in both groups the IO courses between the two bellies of the IR while the SO passes posterior to both bellies of the SR (Figure 1e,f).

FIGURE 3.

The course of superior oblique (SO) (red dotted line) in some cetaceans is shown in internal ventral views. (a,b) SO passes the “functional trochlea” (*) and inserts onto the sclera posterior to the scleral belly (s) of superior rectus (SR) in the Dall's porpoise (Phocoenoides dalli, USNM 564066 R) and bottlenose dolphin (Tursiops truncatus, USNM 571618 R). (c) SO gives off multiple slips in the functional trochlea and inserts onto the palpebral belly of SR (blue), CT (red) as well as onto the sclera (not shown) in the pygmy sperm whale (Kogia breviceps, USNM 594027 R). In Video S3 the CT layer is folded back to reveal the multiple fascicles of the SO scleral insertion. (d) The functional trochlea receives contribution from the MR and strongly holds the SO in the fin whale (Balaenoptera physalus, USNM 594182 R). (e,f) The functional trochlea mainly consists of the thick fascial sheath arising from the medial division of SR and MR in the minke whale (Balaenoptera acutorostrata, USNM 594656 L). These structures are shown in detail in Video S1. Scale bars: 10 mm

3.2. Cetacean SO

3.2.1. Functional trochlea of SO

After passing the “anatomical trochlea” which serves as its pulley in mammals, the SO in all cetacean specimens examined first turned caudally about 45° and then penetrated the ECM and the palpebral belly of SR, passing between the two parts of the latter and emerging medial to the scleral belly of SR (Figure 3a–f). The SO turns another 75° or so caudally during the passage through the SR. The medial part of the SR belly is joined by strong fibrous aponeurosis‐like tissue arising from the MR, ECM, and ICM in some species including the fin whale (Figure 3d) and the minke whale (Figure 3e,f). With or without those additional contributions, the medial part of the SR is positioned to hold the SO tightly at the location of the second and sharpest directional change of the SO muscle. The passage of the SO through these structures thus appears to serve as the “functional trochlea” of the SO in cetaceans (Figure 3a–f, yellow asterisks). This notion, proposed by Weber (1886) and supported by Zhu et al. (2000), finds in the tight apposition of SO, SR, and LR a likely mechanism to allow transmission of force around a sharp bend thus serving as a functional SO trochlea without involving a skeletal attachment or cartilage pulley. Nothing like the functional trochlea of the SO was found in any of the non‐cetacean specimens examined, all of which had their sharp SO direction change at the normal mammalian trochlea.

3.2.2. Primary (scleral) insertion of SO

The primary SO scleral insertions of all the species examined here join the globe in similar locations, with the main variables being how far temporalward the insertion reaches and whether there is a fleshy insertion versus a more tendinous one. The SO bellies as they turn caudally and pass through the FT are very fleshy in cetaceans, and either remain fleshy all the way to the insertions as in most toothed whales or form broad aponeurotic tendons of insertion as in the baleen whales examined here. The fleshy SO insertions in small to mid‐sized toothed whales are shown in Figure 4a,b,d for the melon‐headed whale, Risso's dolphin and pygmy sperm whale. In these types the SO insertions appear to be areas of insertion rather than lines of insertion, though such distinctions will require histological analysis to distinguish the precise attachment zones. The muscular scleral insertions of the oblique muscles in toothed whales contrast with the fibrous (tendinous) scleral insertions found in most of their rectus EOMs (e.g., SRs in Figure 4). The exception is the scleral insertion of the MR in some small to mid‐sized toothed whales where it remains muscular until right at the sclera (Figure 2, not shown in photographs). In contrast to these fleshy insertions, the SO in two baleen whale species examined, fin whale, and minke whale, have broad tendinous insertions as seen in Figure 4c,e,f; Video S1. In these species the SO emerge from the functional trochleae still fleshy and then fan out into broad, flat mostly fibrous tendons that nevertheless still contain visible muscle fibers until becoming solely fibrous at their final curved lines of insertion.

FIGURE 4.

Superior oblique (SO) main insertion onto the sclera and accessory insertions onto adjacent structures in toothed and baleen whales shown in internal ventral view. The fleshy SO scleral insertion is prominent in toothed whales (a,b,d) in contrast to the fibrous scleral SO insertions in baleen whales (c,e,f). (a) SO insertion (1) is only onto the sclera in the melon‐headed whale (Peponocephala electra, USNM 550008 R). (b) SO inserts onto the RB (2) as well as onto the sclera (1) in the Risso's dolphin (Grampus griseus, USNM 594001 R). (c–f) SO inserts onto the sclera (1), RB (2), palpebral belly of SR (3), scleral belly of SR and CT in c, the fin whale (Balaenoptera physalus, USNM 594182 R), (d) the pygmy sperm whale (Kogia breviceps, USNM 572142 L) and (e,f) the minke whale (Balaenoptera acutorostrata, USNM 594656 L). In the latter, elaborate SO insertions onto the palpebral belly of SR (3) are shown, as is the fibrous scleral insertion (1) marked by red pins (e) that attaches proximal (apical) to the scleral belly (s) of SR. This specimen is shown in Videos S1 and S2. Scale bars: 10 mm

In both toothed and baleen species the main SO scleral insertions generally attach beyond the temporal margin of the SR scleral tendon, thus between the scleral insertions of SR and LR on the superolateral sclera, a condition we code as beta (β) in Table 2 (cf. Figures 1e,f, 3b and 4a; Video S2). SO insertions that do not reach past the SR scleral insertion are shown as alpha (α) in Table 2, but were found only in the non‐cetaceans sampled here (below). In a few cetaceans the SO insertion reaches farther around the temporal sclera, gamma (γ) in Table 2, to insert at the level of the superior border of the LR scleral belly. This pattern was found in harbor porpoise and some but not all specimens of Blainville's beaked whale and Gervais's beaked whale (not shown in figures). In all cases the SO insertion is immediately distal to the insertion of the superolateral part of the RB (Figure 4a–d).

3.2.3. SO accessory insertions

In addition to the main insertion onto the sclera, the SO in some cetacean species had additional attachments onto other nearby structures. Combinations of AIs vary among species as shown in Table 2, and include attachments to (1) the fibrovascular connective tissue layer (CT) that surrounds the RB and sometimes spreads onto the otherwise bare sclera, (2) the distal margin and more proximal surface of the retractor bulbi (RB), and (3) parts of adjacent rectus muscles, most often to the palpebral belly of SR (SRp). The simplest case is when the SO inserts onto the sclera and barely spreads onto the CT and RB as in melon‐headed whale (Figure 4a). Next are cases with greater spread onto the surface of the RB as shown in Risso's dolphin and fin whale where the broad and fleshy scleral insertions extend along the surface of the RB (Figure 4b,c). In a few species SO insertions spread onto multiple structures on different planes or levels. The most elaborate multiple insertions of SO were found in specimens of pygmy sperm whale and minke whale (Figure 4d–f; Videos S1 and S3). The SO insertion onto the palpebral belly of SR was especially muscular and strong in these species. Pygmy sperm whale specimens had two additional interesting aspects. In one (K.breviceps USNM 593969 R) there was a narrow muscle slip between SR and LR, and the SO inserts onto this slip medially, instead of onto the palpebral belly of SR/LR as in other K.breviceps specimens. And notably, the SO insertions in these Kogia specimens reached farther toward the apex along the surface of the RB than in any other cetaceans examined (Figure 4d). This provides a more “posterior” SO attachment that possibly assists leverage for infraduction (see discussion).

From least complex to most complex, the SO can insert onto nothing but sclera, as in many smaller odontocetes, or can insert onto the sclera and have extensions onto essentially every surrounding structure, including RB, CT, palpebral, and scleral bellies of SR and LR as in the minke whale. However, even at their most elaborate (Figure 4d–f) the SO AIs are never as complex as those of the IO in the same species (below).

3.2.4. Main and accessory SO insertions in non‐cetaceans

It is important to note that the insertion of the SO to the posterior surface of the globe as in humans is rare among mammals and may be restricted to some species of primates. In all the non‐cetacean species examined here the SO insertion is in the temporal part of the anterior hemisphere of the globe, as in cetaceans. The temporal reach of the SO insertion in most of the non‐cetaceans was less than in the cetaceans, namely not reaching past the SR tendon (Table 2, α). These included deer, goat, manatee and others. The SO insertions in pygmy hippo and Florida panther were past the SR and in panther reached the LR tendon (Table 2, β; Figure 5b), while in Asian small‐clawed otter the SO insertion crossed the level of the LR insertion (γ; Figure 5c). In most of the aquatic and terrestrial mammals examined here, the SO has only a single insertion that attaches to the sclera. An accessory SO insertion onto the scleral belly of SR was found in some terrestrial mammals such as white‐tailed deer (Figure 5a, asterisk), raccoon, woodchuck, and gray squirrel (Table 2).

FIGURE 5.

Superior oblique (SO) insertions in some non‐cetacean species. (a) In the white‐tailed deer (Odocoileus virginianus, KM001, R), the SO tendon fans out, inserting to the sclera and fine SO fibers attach to the global layer of SR (black asterisks, SR is reflected). (b) SO tendon splits into two and runs anterior and posterior to the global layer of SR (reflected) in the Florida panther (Puma concolor, USNM 602250, L). Anterior insertion onto the sclera (1) is continuous with the global layer of LR while the posterior tendon (2) inserts onto the sclera at the lateral edge of the global SR insertion. The image was flipped horizontally to match the orientation of the adjacent images. (c) SO tendon (1) runs between the global and orbital layer of SR in the Asian small‐clawed otter (Amblonyx cinereus, NZP 114768, R). 1, SO anterior tendinous insertion; 2, SO posterior tendinous insertion; black asterisk, fibrous connection between SO and global layer of SR; red asterisk, scleral insertion of SO in deer. Scale bars: 5 mm

In Florida panther the SO tendon splits into two after passing the cartilaginous trochlea (Figure 5b). The two SO tendons straddle the scleral insertion (global layer) of SR. The one anterior to the scleral insertion of SR continues temporally and blends with the global insertion of LR, while the posterior tendon inserts directly to the scleral at the temporal edge of the SR insertion. In the Asian small‐clawed otter (Figure 5c), the SO becomes tendinous after passing the trochlea, runs between the global and orbital layer of SR and inserts onto the sclera deep (ventral to) the LR in the anterior hemisphere of the globe, close to the eyelid. The relation of the SO to the two insertions (global and orbital) of SR in this specimen resembles that of the IO to the two bellies (scleral and palpebral bellies) of IR in cetaceans. In this case it is because the SO inserts so far anterior that it cannot cross deep to the global insertion of SR.

3.3. Cetacean IO

3.3.1. Functional trochlea of IO

In all cetacean specimens examined, the IO departs from its laterally oriented origin along the anteroventral (maxillary) margin of the orbit, then turns caudally and penetrates the ECM and the palpebral belly of the IR (Figure 6a–f), which hold the “turn” of the IO in the manner of a functional trochlea, as seen with the SO. The functional trochlea (FT) wraps the IO against the palpebral belly of IR just medial to the scleral belly of IR. During this passage, the IO changes direction about 120° from its origin (Figure 1a,c). The FT consists of a medial band of the palpebral IR and in some cases a piece of the ICM that passes between IR and MR. As was seen with the SO, the circular ICM fibers run parallel with the IO in the region of the IO FT (Meshida et al., 2020). The size of the IO FT varies among species and individuals. In some toothed whales such as Risso's dolphin, bottlenose dolphin and Fraser's dolphin, the functional trochlea is relatively large (Figure 6c–e, yellow asterisks). In contrast, while one minke whale specimen (B.acutorostrata, USNM 504674) has a relatively large functional trochlea of the IO (Figure 7e,f), the IO FT is merely a narrow muscle slip in a different individual of the same species (USNM 593554 R; Figure 6f), though the latter may have resulted from damage during dissection. This suggests that there are individual variations in the size of the FT, likely a difference in how much of the medial part of the palpebral IR is involved.

FIGURE 7.

Close‐up images of inferior oblique (IO) main and accessory insertions in toothed and baleen whales. The scleral bellies of IR in c–e were severed to demonstrate the intricate accessory IO insertions occurring between the two bellies of IR. (a) The distal end of IO as it emerges from the IR and inserts into the sclera is shown in the Risso's dolphin (Grampus griseus, USNM 594001 R), (b) IO fans out after coming out of the two bellies of IR and anchors onto the RB, CT and the sclera (1) in the melon‐headed whale (Peponocephala electra, USNM 550008 R). Accessory insertions onto the CT, scleral belly of IR, palpebral bellies of IR and LR are present in this specimen but not shown in the image. (c) IO insertion on the scleral belly of IR (3) extends to the conjunctiva (not shown) in the pygmy sperm whale (Kogia breviceps, USNM 594027 L). (d) Numerous insertions onto the palpebral belly of IR are present in the fin whale (Balaenoptera physalus, USNM 594182 R). (e,f) IO gives off multiple slips between the two bellies of IR in the minke whale (Balaenoptera acutorostrata, USNM 504674 R). The insertions onto the palpebral belly of IR (2) are particularly prominent. The orientation of the specimens is internal ventral view. IO insertions onto 1, sclera; 2, palpebral belly of IR; 3, scleral belly of IR; 4, RB; 5, CT; 6, conjunctiva; 7, scleral belly of LR (partly shown); asterisk, functional trochlea of IO; Yellow dashes, cut edge of the scleral belly of IR. Blue dashes indicate scleral insertion of IO. Scale bars: 5 mm in e, others, 10 mm

After coming out of the functional trochlea, the IO passes between the scleral and palpebral bellies of the IR (Figures 6c–f and 7b–f). This passage likely adds to the role of the functional trochlea in constraining the IO insertion path. Together, these structures appear to help support a smoothly arced course for the IO as it passes from nasal to temporal ventral to the globe. The IO emerges from the IRp/IRs space to finally reach its main insertion site on the temporal sclera (Figure 7a–f #1). The scleral bellies of IR are shown intact in Figures 6c,f and 7b and shown severed (yellow dots) to reveal the course of the IO in Figures 6d,e and 7c–f.

3.3.2. Primary insertion of IO

The positions of the main IO scleral insertions noted above for bottlenose dolphin and minke whale are typical for most of the cetaceans examined here; namely, a curved line or broad area of insertion onto the sclera immediately distal to the RB insertion and just temporal of the scleral tendon of IR (Figure 1e,f; Table 3, α). This location is on the anterior (external) hemisphere of the globe, near the equator. Note that the IO scleral insertions are not visible in Figures 7b–f and 8b,c since the dissections were intended to demonstrate the accessory IO attachments. The distal extent of IO insertions relative to the scleral bellies of IR and LR are indicated in Table 2 as type alpha (α): between IR and LR, beta (β): just reaching LR, and gamma (γ): reaching the superior edge of LR. Almost all the cetaceans are in the alpha category as shown in Figure 1e and Video S2. The main IO insertions in Blainville's beaked whale are located farther temporalward than in the others, reaching to the LR tendon (Table 3, β). None of the cetaceans have IO insertions that reach past the LR insertion as seen in some non‐cetaceans (below).

TABLE 3.

IO insertion patterns in cetaceans and non‐cetaceans

FIGURE 8.

Close‐up images of inferior oblique (IO) insertions show the main scleral insertion and intricate accessory insertions onto the adjacent structures in some toothed whales. The scleral belly of IR was severed in a and c (yellow dotted lines) to show the accessory insertions of IO. a–c are all in internal ventral view. (a) The internal surface of IO and the scleral belly of IR are continuous via fibrous connective tissue (3) in the Risso's dolphin (Grampus griseus, USNM 594001 R). (b) The functional trochlea of IO (asterisk) is partially severed to show that the IO gives off multiple insertions onto the surrounding structures in the melon‐headed whale (Peponocephala electra, USNM 550008 R). (c) IO gives off multiple slips between the palpebral and scleral bellies of IR and the functional trochlea of IO in the pygmy sperm whale (Kogia breviceps, USNM 594027 R) These insertions are shown in Video S3. IO insertions onto: 1, sclera; 2, palpebral belly of IR; 3, scleral belly of IR; 4, RB; 5, CT; 6, conjunctiva and the palpebral belly of IR (inferior part). Yellow dots: cut edge of the scleral belly of IR. Blue arrow points to IO scleral insertion lying out of the image frame. Scale bars: a, 10 mm; b,c, 5 mm

Like their SO scleral insertions, the IO scleral insertions in odontocetes are generally fleshier (Figure 8a) and those in mysticetes generally more tendinous. The main insertion onto the sclera is wide and muscular at the insertion point in all the odontocetes examined, with the pygmy sperm whale being the extreme case of a broad and muscular IO insertion (Figure 8c). In contrast, minke whale and fin whale have IO insertions that are broad but very tendinous rather than fleshy (Video S2). Nevertheless, the main muscle belly of IO in those species is cylindrical and fleshy as in other cetaceans.

3.3.3. IO accessory insertions

In addition to the main insertion onto the sclera, the IO inserts onto multiple other structures in most cetaceans examined (Table 3). AIs of the IO were both more common than those of the SO and more complex in nature. Non‐scleral attachment sites included: the distinctive CT surrounding RB; the RB itself; the palpebral bellies of IR and LR; the scleral insertion tendons of IR and LR; and finally, the conjunctival gland and deep surface of the conjunctival epithelium (Table 3, first column). The combination of multiple IO insertions varies among species and only two species (spinner dolphin, rough‐toothed dolphin) appeared to lack any accessory IO attachments. The accessory IO attachments can be thin and fibrous or quite fleshy. In the best‐developed cases the IO fans out with numerous fleshy processes forming an elaborate set of attachments to most surrounding structures as in fin whale (Figure 7d), minke whale (Figure 7e,f) and pygmy sperm whale (Figures 7c and 8c). In the simplest cases, the main IO insertion spreads onto the adjacent CT and/or RB, but other attachments are absent as in the harbor porpoise and Dall's porpoise (not shown). Extension of the main IO scleral insertion onto the adjacent CT and RB were common in our dissections and may turn out to be generally present among cetacean groups. It should be noted that most of the cetacean specimens categorized as lacking any accessory IO insertion were in poor condition except for specimens of harbor porpoise (Phocoena phocoena, USNM 593413 R) and Fraser's dolphin (L.hosei, USNM 594200 R). The left eyes of the Fraser's dolphin (L.hosei, USNM 594200) and the harbor porpoise (P.phocoena, USNM 593413) have multiple insertions (Table 3). Hence it is possible that all the cetacean species examined here actually have accessory IO insertions.

Insertions onto the CT

The connective tissue layer (CT) separating the RB from IR and LR receives a distinctive layer of fibrous connections from the IO in most of the species examined (Table 3). In most cases the extensions from the IO blend smoothly into the CT. The IO attachments to the CT were most prominent in minke whale (Figure 7e,f #5) and pygmy sperm whale (Figure 7c #5 and 8c #5). Such connections were not found in a few species, mostly smaller odontocetes (Table 3, CT), but some of those specimens were of lower quality and may be negative artifacts.

Insertions onto the RB

IO attachments to the RB among toothed whales can vary from only a fibrous sheet connecting them as in Dall's porpoise (not shown), to a strong aponeurotic insertion onto RB as in the melon‐headed whale (Figure 7b) or to broad fleshy extensions of the IO that overlie fairly large areas of the RB as in the pygmy sperm whale (Figure 8c) and Gervais’ beaked whale (not shown). The IO insertions in the minke whale (Figure 7e,f, #4) and fin whale (Figure 7d, #4) were relatively extensive but mainly fibrous rather than fleshy as in the toothed whales.

In most species, as the IO courses between the scleral and palpebral bellies of IR it gives off multiple fleshy or fibrous slips to surrounding structures including the palpebral belly of IR, the scleral belly of IR and the conjunctival gland/conjunctiva. In a few cases the connections to the IR bellies extend farther temporalward, onto the palpebral and/or scleral bellies of the LR.

Insertions onto the palpebral belly of IR

All the cetacean species with significant accessory IO insertions had an IO insertion onto the palpebral belly of IR (Table 3). The complexity and robustness of the connection between the IO and the IR palpebral belly varied and was sometimes quite substantial but was never as strong as the main IO scleral insertion. In some cases, as in the melon‐headed whale (P.electra, USNM 550008 R) and pygmy sperm whale (Kogia breviceps, USNM 594027 L), IO gives off multiple muscle slips while it passes through the palpebral belly of IR, which invests IO medially (Figures 7c and 8b). The palpebral bellies of IR investing the IO in the two images were severed in order to show the IO insertions.

In the minke whale (B.acutorostrata, USNM 504674 R), the IO insertion onto the palpebral belly of IR is muscular and extensive. It blends with the palpebral belly of LR as well (Figure 7e,f, #2). In fin whale (B.physalus, USNM 594182 R), the IO gives off multiple layers of extensive fibrous CT and muscle slips onto the palpebral belly of IR (Figure 7d, #2) as well as onto conjunctiva (Figure 7d, #6).

Insertions onto the scleral belly of IR

Fibrous connections between the sheath of the IO and the scleral belly/tendon of IR were seen in minke whale and in many of the odontocete species examined (Table 3, IRs). In all cases, the IO insertion onto the scleral belly of IR was a non‐muscular extension of the IO sheath. It is shown here for the Risso's dolphin (Figure 8a, #3), melon‐headed whale (Figure 8b #3), and pygmy sperm whale (Figure 7c #3). As with the IO attachment to the palpebral belly of the IR, absence of a connection to the scleral belly of IR may be an artifact of preservation or dissection; likely this attachment is generally present in cetaceans.

Insertions onto the conjunctival gland

As the IO passes between the palpebral and scleral bellies of IR it comes into direct contact with the inferior portions of the conjunctival gland (Figure 2g). In some specimens, fibrous connections bind the IO sheath to the gland and adjacent fascia (Table 3, conjunctiva). These attachments were positively identified in fin whale (Figure 7d, #6), pygmy sperm whale (Figure 8c, #6; Video S3), Atlantic spotted dolphin (USNM 594532 R), Risso's dolphin (USNM 594001 R) and short finned pilot whale (USNM 594045 R). The fibrous IO insertion onto the conjunctiva in the Atlantic spotted dolphin was extensive and covered the conjunctival gland (not shown).

Insertions onto the palpebral belly of LR

In a few species (bottlenose dolphin, melon‐headed whale, and minke whale; Table 3) the accessory IO insertions extend farther temporalward than in the others, allowing attachments to the palpebral bellies of the LR (not shown). These attachments to LR appear as continuations of those to the palpebral belly of IR. It is likely however, that IO insertions onto the palpebral LR may be more common than found in our dissections since many species, especially small odontocetes, have the palpebral bellies of IR and LR strongly fused and this location must be divided in order to reflect the muscles to view deeper structures. This is likely in the case of Blainville's beaked whale (USNM 571927 L) as an IO attachment to the scleral belly of LR was identified in this specimen (below).

Insertions onto the scleral belly of LR

This condition was positively found only in the minke whale and Blainville's beaked whale. The insertion onto the scleral belly of LR in minke whale was fibrous and continuous with the CT sheet connecting to the IR scleral belly (Figure 7e,f, #7). In Blainville's beaked whale (USNM 571927 L), a thick and fibrous CT sheet arose from the IO and merged with the scleral belly of LR (not shown).

The combination of different IO accessory attachments and the robustness of each of them varied among the cetacean specimens examined here. The occurrence of these attachments as listed in Table 3 is biased toward absence of these structures since they can be lost due to factors of preservation and dissection, but not gained. Thus, the presence of multiple accessory IO insertions seems likely to be common among both mysticete and odontocete species. Unfortunately, the exact nature of some of these attachments, whether extensions of the IO proper or of the IO muscular sheaths, cannot be determined without finer methods. Undoubtedly the most elaborate suites of SO and IO insertions in our sample were seen in the two mysticetes, minke whale and fin whale, and perhaps surprisingly in the odontocete pygmy sperm whale Kogia breviceps. In particular, the accessory IO attachments in these species are unparalleled.

3.3.4. Main and accessory IO insertions in non‐cetaceans

Half of the non‐cetacean species examined here had IO similar to those seen in most of our cetacean samples, with muscular insertions just distal to the RB and between the IR and LR scleral insertions (Table 3, α) In a few species the IO extended farther temporally and inserted deep to the global insertion of LR (Table 3, β). The white‐tailed deer was exceptional as the IO inserted onto the sclera at the level of the superior border of LR (Table 3, γ). Accessory IO insertions were found in only one of the 12 non‐cetacean species dissected here; the IO insertion in cottontail rabbit fans out and blends with the CT over the sclera at the lateral edge of the scleral tendon of IR (not shown). Notably, multiple IO insertions were not identified in the other aquatic/semi‐aquatic mammals such as Florida manatee, pygmy hippopotamus and Asian small‐clawed otter (Table 3).

The IO insertion in all the non‐cetaceans examined was confined to the anterior hemisphere of the globe; none were extended to the posterior hemisphere as is seen in humans and other primates. In some species with more anterior IO insertions (goat, skunk, squirrel, raccoon, and Florida panther), it is positively identified that the IO runs between the two layers of IR (i.e., ventral to the global layer and dorsal to the orbital layer of IR).

4. DISCUSSION

The globe, EOMs and circular muscles in cetaceans appear to be much more mobile within the orbital space than those of most other mammals, with translations and rotations of the globe visible in any close observation of live animals. Thus, instead of whales having little or no eye movements as was believed by the eminent anatomist Weber (1886) and others (Duke‐Elder, 1958; Wall, 1942), they likely have more extensive and mechanically complicated eye movements than other mammals. Weber of course had access only to dead specimens and could not observe cetacean eye movements in captive animals or in videos of free‐living animals taken by scuba divers (e.g., search “whale eye” on YouTube).

To make structural comparisons between cetaceans and other mammals we must first introduce a hypothesis that the palpebral bellies of the cetacean rectus EOMs are derived from or homologous to the orbital layer of rectus muscles seen generally in mammals. Detailed consideration of this hypothesis will wait until the third paper in this series which will focus on cetacean rectus EOMs. For the present we will limit the discussion to stating that the relations to surrounding structures is remarkably similar between the orbital layer in human rectus EOMs and the palpebral bellies of the recti in cetaceans, with the difference that the latter are hypertrophically muscular (Figure 1f, p). Thus, the overall architecture of the SO and SR are comparable between human and cetacean (Figure 1f, SR), with similar attachments to each other (SO‐SR), to tenon's fascia and beyond into the palpebral structures if human LPS is included. Notably, in both species the oblique tendon passes ventral/posterior to both the SR scleral (global) and palpebral (orbital) portions. The unusual condition of the cetacean IO running between the two layers of the IR (Figure 1f, IRp, s) differs both from the cetacean SO just noted and from the IO in humans and many other mammals in which the IO passes ventral/posterior to both orbital and global layers of the IO. This requires some clarification to maintain the general consistency of our comparative hypothesis.

In humans and many other kinds of mammals the oblique scleral insertions are located posterior to the global insertions of the rectus EOMs. That is to say, the SO and IO insert farther from the limbus than do the recti (Ottley, 1879; Prangen, 1928; Prince et al., 1960). In these cases, the SO tendon runs ventral to the SR global layer, between it and the globe, and the IO runs ventral to both global and orbital layers of the IR, between the latter and the floor of the orbit. This pattern has been reported for human, monkey, and a variety of domesticated and exotic mammals (Demer, 2017; Prince, 1956; Weber, 1886). However, in six of the non‐cetacean species examined in the present study, either IO, SO, or both do not conform to the human pattern. In goat, skunk, squirrel, raccoon, and Florida panther the IO ran between the global and orbital insertions of the IR on its way to inserting on the superolateral part of the anterior hemisphere. The reason for this in these species is that the IO inserts more anteriorly and the IR more posteriorly compared to humans and similar cases. In the Asian small‐clawed otter we found that the SO courses and inserts extremely far anteriorly, well ahead of the SR global insertion (Figure 5c), such that the SO is running by default between the global and orbital layers of the SR. The anterior part of the split SO tendon in the Florida panther likewise runs anterior to the global insertion of SR and, thus, between that and the orbital SR insertion. Thus, it appears that for both SO and IO the oblique muscle paths run between the two layers of the SR and IR when a far anterior oblique global insertion is combined with a far posterior rectus insertion. This anatomical variability combined with the failure to identify separate global and orbital layers of the recti is likely the reason why descriptions of the position of the IO tendon relative to the IR are so conflicting in the older literature (cf. Duke‐Elder, 1958, p. 545; Prince et al., 1960; Weber, 1886, p.130). The path of the IO between the two layers of the IR seen so clearly in cetaceans (Figures 6c–f and 7b–e) is thus not as unique as it seems and not only highlights the likely variable relations of oblique and rectus insertions across mammals but suggests that the basic plan of ocular suspension may be extremely adaptable for different orbital and global configurations.

4.1. The origins and insertions of SO and IO in cetaceans are broadly typical for mammals

The current results confirm that numerous features of the oblique EOMs in cetaceans including their bony origins, scleral insertions, fascial sheaths, and innervation are similar to those known for many other mammals (Fink, 1962; Hosokawa, 1951; Motais, 1887; Prince, 1956; Weber, 1886). In the general pattern seen in cetaceans and most mammals, the SO and IO cross from nasal to temporal in order to insert on the temporal sclera, with SO passing dorsal to the globe from a trochlea and IO passing ventral to the globe from its maxillary origin. In most species the oblique scleral insertions attach in the anterior hemisphere. Because of these conserved features, the basic actions of the obliques in most mammals appear to be cycloductions of the globe around the optical axis, as is usual in most other vertebrates (Bianco et al., 2012; Wall, 1942). The main variations in mammalian oblique anatomy are not well‐documented but include the anteroposterior positions of the SO and IO scleral insertions relative to rectus EOM insertions and the angles between these two groups of attachments (Prangen, 1928; Prince et al., 1960). From these studies it seems that the main functional variations of oblique EOMs emerge when their insertions extend across additional rotation axes of the globe and allow significant vertical and horizontal actions in addition to their basic torsional ones, with frontal‐eyed, foveated primates highlighting this aspect.

Mostly lacking, however, are comparative studies that distinguish global and orbital layers and that document accessory origins and insertions. Consideration of all the attachments of EOMs in comparative context has only ever been attempted by Motais (1887) and only with limited comparative depth. He described the eye muscles in a selection of fishes, amphibians, reptiles, birds and mammals and illustrated accessory EOM attachments in horse, cattle, and humans including ones between SO, SR, LR, and IO. He included discussion of different types of accessory attachments and emphasized their likely functional importance in the respective species. The synthesis of anatomical and surgical knowledge of oblique muscles by Fink (1962) included none of the comparative detail seen in Motais but clearly expressed a view of human oculomotor structure that saw mechanical consequences for all of the finely graded and positioned attachments between EOMs, their sheaths, and Tenon's fascia. Only in recent decades have the orbital and inter‐EOM connections come to the fore, mainly through the functional demonstration that the connective tissue suspensory apparatus forms a system of mobile EOM pulleys (Demer, 2007, 2017; Miller, 1989). This spurred histological and immunohistochemical studies that revealed the full structural plan of the ocular suspension (Demer et al., 1995, 2003; Kono et al., 2002, 2005).

The present study is mainly concerned with identifying all the primary and accessory attachments of the oblique muscles in cetaceans to begin allowing comparisons with contemporary views of mammalian orbital mechanisms. These comparisons necessarily involve currently untestable hypotheses about cetacean eye movements. Although there do not appear to be any modern published studies of eye movement measurements in cetaceans it nevertheless seems a safe speculation that well‐controlled three‐dimensional eye movements are necessary for accomplishing the visual tasks that are evident in the cetacean behavioral literature (reviewed in Mobley & Helweg, 1990; Perrin et al., 2009; Wells, 1999) and in the informal documentation of contemporary videography of captive and wild cetaceans (online search e.g., “whale eye close‐up”). In the absence of formal oculography our interpretations of functional attributes of cetacean eye muscles are purely based on structural comparisons with better‐studied mammalian species and as such are hypothetical and intended to identify functionally testable features of cetacean ocular motor anatomy.

4.2. Basic variations in cetacean SO and IO scleral insertions are: Muscular versus tendinous, extent of temporal reach and extension to the posterior hemisphere

Cetacean oblique scleral insertions vary from compact muscular insertions lacking any obvious tendon as in some smaller odontocetes (Figures 4a and 7a) to broad tendinous insertions that appear almost as “aponeuroses” (Figures 4e and 8c). Aside from the IO insertions generally being fleshier than SO and both SO and IO being more fibrous in baleen whales there does not seem to be any obvious taxonomic pattern. Differences in muscular versus tendinous insertions are likewise the main variations seen among domesticated animals with fleshy insertions much more commonly seen with IO than with SO (Prince, 1956). Biomechanical studies of SO and IO insertions in humans (Shin et al., 2012, 2013) need to be extended to non‐primates before systematic or functional analyses are available to interpret the significance of fleshy versus tendinous global attachments.

Another basic variable of oblique EOMs is how far the main insertions reach temporally on the globe and how far the main insertions reach posteriorly on the globe. Extension of the SO and/or IO scleral insertions onto the posterior hemisphere of the globe is perhaps the most interesting functional feature missing in cetaceans and most other mammals. The posterior insertions are best developed in humans and are the basis for the increased roles of oblique EOMs in vertical eye movements seen in humans and other primates (Fink, 1962).

4.3. Functional trochleae of SO and IO

Complex functional trochleae of oblique muscles (Figures 3, 4d,e, 6c–f and 7c,e,f; Videos S1 and S2) are unique to cetaceans and relate to their unusual orbital geometry as well as to the presence of huge palpebral bellies of rectus EOMs and extensive striated circular muscles. The paths and structures we refer to as “functional trochlea” were first described by Weber (1886) in a few odontocete and mysticete species and more recently by Zhu et al., (2000), though not by that name. The FT in humpback whale are shown in photographs but not mentioned by Rodriquez et al. (2015). Both SO and IO in cetaceans make sharp turns as they pass through the layers of circular muscles, rectus muscles and Tenon's fascia along their paths to their temporal global insertions. Distinct portions of the palpebral EOM bellies and ICM comprise the reinforced passages that hold the SO and IO sheaths in place at the turns. The FTs and sheaths thus form tunnels within which the SO or IO muscles can move during actions. In the more elaborate cases (Figure 8; Videos [Link], [Link], [Link]), as soon as the muscles emerge from their FTs, the muscle sheaths give off fibrous attachments to surrounding structures. Together with the AIs, the “functional trochleae” of SO and IO clearly contribute significantly to changing the muscles’ directions of pull by being strongly held by the medial part of the SR and IR, respectively. Depending on the actions of the rectus and circular muscles it is even possible that the functional trochleae may also function as “pulleys” for the obliques in cetaceans by adjusting their pulling directions at different positions of gaze. Well‐developed functional trochleae or similar structures were not found in the non‐cetacean species dissected in this study, nor were extensive accessory oblique insertions. These together may be a modification to adjust to the fully aquatic environment, which required changes of palpebral structure, locomotion and likely ocular motor behavior.

4.4. AIs of SO and IO are common in cetaceans, and some of them are among the most elaborate EOM insertions known in mammals

As described in the results, AIs of the SO and IO are numerous in cetaceans and are sometimes clearly present in other species as well (Tables 2 and 3). Among the cetaceans, the variety of IO multiple insertion patterns was greater than those of the SO. The tissue and texture of AIs of cetacean oblique EOMs vary considerably: from fibrous extensions of muscle sheaths (Figures 7c and 8a–c), to highly divided fleshy tendons (Figures 4d and 7d), to broad, partially fleshy aponeuroses (Figures 4e and 8c). Weber (1886) briefly mentioned multiple insertions of IO and SO in the Northern bottlenose whale, blue whale, and some smaller baleen species, while Hosokawa (1951) mentioned a split IO insertion in a fetal sei whale (Balaenoptera borealis). Multiple insertions of the obliques are particularly well developed and even massive in pygmy sperm whale (K.breviceps), minke whale (B.acutorostrata) and fin whale (B.physalus). At first inspection, the minke whale SO AI to the SR palpebral belly (Figure 4e) appears to be a simple aponeurosis. But closer views show that it is a muscular fanning‐out of the distal/lateral edge of the SO as the latter emerges from the functional trochlea at the opening in the SR (Video S1). In all three species the IO insertions onto the palpebral belly of IR are as muscular as the main IO insertion onto the sclera (Figures 7d,e and 8b,c). The insertions onto the scleral bellies of IR are extensions of the IO sheaths in these species, and the muscle fibers strongly adhere to the connective tissue sheaths, which is different than in humans. Although their complex patterns of insertion are similar, these species are not alike in taxonomy or ecology: Kogia is a deep diving odontocete, while the latter two species are mysticetes that live mostly above 100 m in water depth. Without knowledge of the eye movements in these species the functional reasons for their extremely well‐developed multiple oblique insertions are not clear, but presumably in each species these attachments have functional consequences.

4.5. AIs of oblique EOMs in humans and other mammals

Knowledge regarding AIs of oblique muscles onto structures other than the sclera comes mostly from human surgical literature, with only a few studies of non‐human mammals describing these structures. The only descriptions we find of accessory oblique attachments that approach the robustness and elaboration seen here in cetaceans are those of horse and cattle figured by Motais (1887, pl. IX bis) as well‐formed connections joining IO and SO with LR, SR and RB. Prince et al. (1960) reported AIs between SO, IO and RB in cattle including a direct connection between SO and IO as we found in deer. However, they reported none of the accessory connections described by Motais for horse, thus, further studies of large cetartiodactyls will be needed to test whether accessory oblique attachments are a common feature of this group.

In humans, monographs and articles intended for strabismus surgeons by Howe (1907), Fink (1962) and others provided descriptions of intricate connections via trabeculae‐like fibers between the oblique muscle fascial sheaths and the surrounding structures, as well as stronger connections between SO and IO tendon fascicles and adjacent rectus EOMs. More recent histological studies provide detailed looks at these connections, especially those between SO and SR and those between IO and IR (Demer et al., 2003; Kono et al., 2005). The SO tendon is surrounded by fascial sheaths consisting of the medial expansions of the sheaths of the LPS and SR as well as expansions of the fascia bulbi (Tenon's capsule). Along with the insertion of the orbital layer/sheath of SO onto the SR pulley, this region of the suspensory apparatus provides important functional coupling between SO and SR (Demer, 2007; Kono et al., 2002, 2005).

Ventrally in the orbit, the firm connection between the fascial sheaths and orbital layers of the IO and IR at the point of crossing of the muscles provides similar functional coupling between obliques and recti (Demer et al., 2003). The fascia in this region consist of a blending of the sheaths of IO and IR with ventral expansions of the sheaths of LR and MR to form the suspensory ligament of Lockwood, supporting the globe like a hammock and including the functional pulleys of both IR and IO (Demer, 2009, 2017; Fink, 1962).

Along with these basic connections of the human obliques, there are numerous variations reported for both SO and IO. The SO scleral insertion shows a wide range in its location, orientation, width, and subdivisions (Demer, 2017; Fink, 1962), and occasional accounts of secondary or aberrant connections between LPS, SR and SO are reported (Fink, 1962; Kocabiyik, 2016). IO variations are more numerous, including multiple divisions in the scleral insertion (Paik & Shin, 2009; Yalçin & Ozan, 2005), muscular bridges between the IO and IR (Yalçin et al., 2004) and double bellies of the IO (De Angelis & Kraft, 2001; De Angelis et al., 1999). Many of the fascial attachments of tendons and sheaths of human oblique muscles and the variations of their scleral insertions are thus comparable in anatomical location to the AIs of the oblique muscles in cetaceans. Other structures that may influence oblique muscle actions also seem directly comparable between humans, cetaceans, and other mammals. For example, the neurofibrovascular bundle that bridges the IO and IR and contains the oculomotor nerve branch to the IO in humans (Stager, 1996; Stidham et al., 1998) is similar to the cetacean accessory IO fascicles that attach to the IR and are accompanied by the nerve to IO and blood vessels (Figure 8a,c).

4.6. Compartmental organization of mammalian EOMs

The extreme breadth of SO and IO insertions in some cetacean species (Figure 4e,f; Video S2) suggest possible anatomical and biomechanical compartmentalization as has been demonstrated in humans and some domesticated mammal EOMs. These studies have demonstrated that the EOMs in humans and other mammals are structurally and functionally compartmentalized in two manner: (1) global versus orbital layers, and (2) medial versus lateral portions (reviewed in Demer, 2015, 2017). EOM compartmentalization is found in both rectus and oblique muscles where compartments differ in muscle fiber types, innervation from intramuscular motor branches and in spatially and biomechanically distinct insertions (da Silva Costa et al., 2011; Demer, 2015). The combination of quantitative analysis of MRI functional imaging and 3D reconstruction of orbital and intramuscular histology provide functional and anatomical evidence of EOM compartmentalization and show that the ultimate lines of action of each muscle are continuously influenced by both kinds of compartmental organization. These results are worth considering in some detail as they provide a model for future comparative studies of oblique EOMs.

Functional anatomical studies using high resolution MRI have demonstrated that compartments within EOMs show contractile differences during a variety of eye movements (reviewed in Demer, 2015, 2017). While many of the studies focus on rectus EOMs, differential contractility within the SO has been demonstrated during infraduction and vertical fusional vergence (Clark & Demer, 2016; Demer & Clark 2015). The mechanical independence of EOM compartments was demonstrated by passive and active stimulation of bovine EOMs (Shin et al., 2012, 2014) and tendon compartments (Shin et al., 2013) as well as by pathological studies on selective unilateral EOM palsies (Shin & Demer, 2015; Suh et al., 2016). The parallel EOM fibers continuing as broad tendons can allow different parts of a muscle to have substantially separate scleral insertion sites, and thus, different oculorotary functions (Demer, 2015). The continuity of different muscular compartments with distinct parts of the orbital and global insertions of oblique EOMs has been shown anatomically in humans and monkeys where Le et al. (2015) reported that the SO lateral/superior compartments insert onto sclera posteriorly while the medial/inferior SO compartments insert anteriorly; the posterior insertions have mechanical advantage for infraduction and the anterior, for incyclotorsion. It is not clear how many mammalian taxa have species in which oblique EOMs have significant vertical roles as the eye muscle anatomy in only a few taxa have been measured in rigorous enough manner (e.g., Cox & Jeffery, 2008; Ezure & Graf, 1984) and in vivo MRI studies are lacking.

4.7. Possible functions of the broad scleral insertions, AIs, and functional trochleae of cetacean oblique muscles

The demonstration of compartmental division of biomechanical action within individual EOMs in multiple species thus suggests that parallel muscle fibers spreading coherently into broad scleral insertions may be a generalized anatomical basis for compartmentalized EOM function in mammals (Shin et al., 2012, 2014). The broad oblique insertions and the way parallel bundles of muscle fibers spread within them make it possible that the cetacean SO and IO are similarly compartmentalized (Figures 4b–f and 8c; Videos [Link], [Link], [Link]). The location of their scleral insertions on the anterior hemisphere and their orientation oblique to those of the recti and more parallel to the optical axis define the basic actions of cetacean obliques as primarily torsional. However, the great width of the SO and IO scleral insertions in some species suggests possible functional differentiation between the anterior/lateral (toward the periphery) and posterior/medial (toward the apex) portions of their scleral insertions. The more anterior parts of the insertions would act torsionally while the more posterior/apical part of the SO tendon should contribute to infraduction and that of the IO tendon to supraduction.

In addition to the plausible biomechanical divisions within the main scleral insertions it appears that the accessory oblique insertions in cetaceans may augment this functional differentiation in additional ways, depending on their sites of attachment. One role would be to extend the basic anterior‐posterior compartmental function of the main scleral insertions. For both SO and IO, AIs that spread onto the RB and CT (Figures 3d, 4b,c, 7b–f and 8c) extend the overall muscle insertions posteriorly, and thus may provide more vertical action. AIs of the IO that spread to the conjunctiva extend the IO attachment anteriorly and should contribute to better leverage for cyclorotary action.

Other roles are likely for accessory oblique insertions onto recti, especially those onto the palpebral bellies of the SR and IR. The connection between oblique tendons and rectus palpebral bellies (SO with SR & LR; IO with IR & LR) are poorly developed in smaller odontocete species (e.g., Harbor porpoise) but are highly developed in the two mysticete species examined as well as in the pygmy sperm whale (Figures 4d–f, 7c–f and 8b,c). These attachments may be able to constrain the pulling directions of the SO and IO main scleral insertions. This would be the familiar role of secondary attachments that maintain the pulling vector of a muscle by resisting the tendency for the muscle's own contraction to displace the pulling path.

Finally, there are the possible effects the oblique AIs have on rectus actions, possibly similar to roles played by the oblique orbital insertions onto rectus pulleys in humans (Demer, 2017, 2019). Since fibers of the ICM strongly attach to the palpebral bellies of the rectus EOMs they need to be included when considering functional connections between obliques and recti.

4.8. The actions of SO and IO on the rectus EOMs may be augmented by the ICM

While the SO and IO have diverse focal attachments with the rectus EOMs, the ICM, a striated circular muscle system peculiar to cetaceans, is broadly attached directly to the recti. The ICM is present in most species we examined as distinct circumferential bands of muscle on the deep surfaces of the palpebral bellies of the SR and IR muscles, with extensions to the LR, but usually no coverage of the MR (Meshida et al., 2020). The ICM fibers run at right angles to those of the recti and are parallel to those of SO and IO when adjacent. The ICM is very slight in some smaller specimens and in ones of all sizes that are not well preserved. It is best developed in mysticetes and larger odontocetes, most notably in the sperm whale where it forms a complete ring separating the rectus EOM bellies from the deeper‐lying RB muscle (Meshida et al., 2020, figures 6 and 9–11; Video 3). The in‐situ relationship between the ICM and the oblique muscles has been difficult to determine and is yet not fully clear. Studies of the oblique EOMs for the present publication have led to realization that portions of the ICM have close connections with the functional trochleae of the obliques and run for most of their extents parallel to adjacent fibers of the SO and IO muscles. Together, oblique insertions and the ICM form a complete muscular belt around the anterior hemisphere of the globe with the ICM also extending back deeper into the orbit. When the cetacean rectus muscles that are shown splayed out in the figures are returned to their normal positions it becomes clear that much of the ICM is adjacent to and has fibers running parallel with those of the SO and IO.

Together then, the obliques and the ICM in cetaceans appear to form a muscular torus that surrounds the globe anterior to the equator and applies torsion not only to the globe but also to the rectus muscles. Interestingly in sperm whale where accessory oblique insertions are poorly developed, there is the most extensive ICM seen among cetaceans, the only ICM forming a complete ring and having two layers medially (Meshida et al., 2020). Since the palpebral rectus bellies have massive attachments to the rather rigid eyelids it seems possible that torsion force applied by the SO, IO, and ICM rotates the globe along with the surrounding rectus scleral tendons inside a shell formed by the palpebral rectus bellies which would resist the cyclorotation in either direction, perhaps tensioning the system for smoother performance.

4.9. The cetacean ICM may derive from the oblique EOMs

The main portions of the ICM extend along the interface between oblique EOMs and palpebral bellies of rectus EOMs, with strong connections to the structures where the obliques penetrate or attach to the rectus muscles, that is, at the functional trochleae and the peripheral AIs of SO and IO (Video S2). These are therefore locations where the outer fascia and fibers of oblique EOMs are joined to the outer layer of the rectus EOMs. When compared with the organization of mammalian EOMs generally (da Silva Costa et al., 2011; Demer, 2015; Khanna & Porter, 2001; Le et al., 2014, 2015) it thus seems possible that the ICM derives from the orbital layer of the oblique muscles. In this view the ICM is comparable to the regions of attachment between the oblique pulleys (orbital layers of SO and IO) and the rectus pulleys (orbital layers of SR, LR and IR) described in humans (Demer et al., 1995, 2003; Miller et al., 2003; Kono et al., 2002, 2005). The postulated modifications of the cetacean SO and IO orbital layers would be hypertrophy and extension of striated muscle fibers farther into the pulley structures and palpebral fascia than in humans. The resulting novel striated muscle components in cetaceans are thus the ICM from the oblique orbital layers and the “palpebral muscle” from the rectus orbital layers. Development and innervation of the ICM is not yet known, but such information should be able to clearly support or refute this hypothesis. The prediction is that ICM portions forming on the deep surface of SR should be direct extensions of the SO, and those on the IR should derive directly from IO. The portions connecting to the LR should be later extensions from both sources. Innervation of the ICM is predicted to derive from both the IO branch of the inferior division of CN III and from branches of CN IV emerging from the SO. At present these surmises are purely hypothetical and await new embryological and neuroanatomical studies for testing.

4.10. The structure of the oblique muscles and the MR suggest they are the main rotators of the eyeball in cetaceans

In some cetaceans, especially small to mid‐sized odontocetes, the SO, IO, and MR appear generally more muscular than the scleral portions of the other rectus EOMs (Figure 2), suggesting that they are more involved in routine eye movements like counter‐rolling during locomotion and directing gaze nasally and ventrally for better “frontal” viewing (Mass & Supin, 2009; Mobley & Helweg, 1990). Steady, forceful action by SO is likely necessary for spy‐hopping behavior where cetaceans erect their heads above the surface of the water to observe the surroundings (Pitman & Durban, 2011). When the head is above the water, the SO presumably rotates the pupil inward and downward so that correspondence between retinal and visual horizons can be maintained. One critical ocular goal during visual searching is likely to be aiming of the temporal area of higher retinal acuity (Lisney & Collin, 2019; Mass et al., 2013) at relevant parts of the visual scene located ventronasally, thus likely requiring robust MR activity along with SO.

4.11. Cetacean oblique EOMs are well‐suited to drive oscillatory torsional eye movements to compensate for head pitch during locomotion

Admittedly, it is a great leap from dissecting eye muscles in museum specimens to inferring function in a taxon that has never been studied in an oculomotor laboratory. Nevertheless, the breadth and depth of eye movement research over the past century allows ample room for informed speculation regarding the consequences of ocular, orbital, and neuromuscular structure/function features. As shown by the present anatomical data, cetacean SO and IO clearly form a suitable agonist‐antagonist configuration for driving ocular rotations about the optical axis (or axes, Lisney & Collin, 2019; Mass & Supin, 2009). With laterally oriented orbits, cycloversions are well‐suited for a role in visual stabilization by producing ocular counter‐rolling during up and down “pitch” movements of the head during swimming. The kinematics of vertical head movements during cetacean swimming show that head movements are much reduced in amplitude compared to the vertical movements of the tail fluke and that they are synchronized in direction and tempo with the latter (Fish & Rohr, 1999; Fish et al., 2003). Unfortunately, there does not seem to be published data on the angular ranges of cetacean head movements suitable for estimating the amplitude of ocular counter‐rolling. Nevertheless, the apparent effective minimization of head movements during cetacean swimming (Fish et al., 2003) suggests that moderate amplitude oscillatory cycloductions matched in frequency and phase with the flexion‐extension movements driving the body forward would result in appropriate compensatory torsional eye movements to stabilize vision during steady swimming. Cetaceans locomote freely in three dimensions thus other EOM functional pairs, possibly including SO and IO participation via AIs, would compensate for other axes of head movement (i.e., yaw, roll), generally in the same range of frequencies and amplitudes as the pitch axis (Kandel & Hullar, 2010).

Eye movements that are compensatory to head movements are generally conceived as involving vestibulo‐ocular reflexes (VOR) using sensory feedback from canal and otolith organs to drive appropriate eye movements. With self‐generated head movements, however, feed‐forward signals using efference copy of the axial motor commands may be a more likely neural mechanism for initiating and synchronizing appropriate eye movements during locomotion (Chagnaud et al., 2012; Straka & Chagnaud, 2017). The thesis that self‐generated motor commands should be the basis of compensatory eye movements during natural locomotion is supported by data from amphibians (Straka & Chagnaud, 2017) and humans (Dietrich & Wuehr, 2019; Dietrich et al., 2020) Eventually, investigations of eye movements and locomotion within cetartiodactyls including bovids, hippos, and cetaceans may give evidence of correlated changes in EOM structure and brainstem motor control during the transition from terrestrial to aquatic locomotion.

5. CONCLUSIONS AND TOPICS FOR FURTHER TESTING

A primary need in future studies is for fine dissections of many more specimens and species of both cetaceans and other mammals to determine the range of variability of main and accessory attachments of oblique EOMs. Imaging studies to precisely locate the scleral and orbital EOM insertions with reference to globe and orbit will likewise be necessary for comparing oblique function across groups. Examination of the relative antero‐posterior positions of the global insertions of obliques and recti and of the position of the oblique tendons with respect to the global and orbital layers of the rectus EOMs would clarify the general patterns of SO and IO paths in mammals. For cetaceans, dissection techniques will be needed that can allow the entirety of individual EOMs to be removed intact for study of possible structural, histological, and innervation compartments. In particular, with complex SO and IO, as in minke whale or pygmy sperm whale, the continuity and histological structure of fibers forming the main and accessory insertions should help determine whether global/orbital and medial/lateral compartmentalization are valid descriptors for cetacean oblique EOMs. The structures with perhaps greatest interest for comparison with human data are the connections between the SO and the palpebral belly of the SR (Figure 4d–f), and between the IO and the palpebral belly of IR (Figures 7c–f and 8b,c) since these attachments may be directly comparable to the insertion of orbital layers of SO and IO onto the system of rectus pulleys in humans (Demer, 2017). Testing for possible common embryological origins between cetacean oblique muscles and the adjacent striated ICM layer will likely require microscopic and immunohistological studies in fetal, juvenile, and adult stages.

AUTHOR CONTRIBUTIONS

Keiko Meshida: concept/design, acquisition of data, data analysis/interpretation, drafting the manuscript. Stephen Lin: acquisition of data (MRI), data analysis/interpretation, critical revision of the manuscript. Daryl P. Domning: critical revision of the manuscript. Paul Wang: acquisition of data (MRI), data analysis/interpretation, critical revision of the manuscript. Edwin Gilland: concept/design, data analysis/interpretation, drafting the manuscript, critical revision of the manuscript, and final approval.

Supporting information

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Meshida K, Lin S, Domning DP, Wang P, Gilland E. The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions. J Anat.2021;238:917–941. 10.1111/joa.13347

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.