Figure 5. Summary of the 3D relationships between BMO, the ASCO and PSCO, the Optic Nerve, the Central Retinal Vein, the Central Retinal Artery and the Long Posterior Ciliary Arteries (with and without their intrascleral branches) as visualized within the 3D Point Clouds from a Representative Normal Rat Eye.

The relationship of BMO (red), ASCO (light blue) and PSCO (purple) relative to the optic nerve (green) can be seen from a clinical orientation in panel (A) and longitudinally in panel (B). In panels (C) - (F) the ASCO and PSCO Point clouds have been turned off and the CRV, CRA and LPCA point clouds have each been turned on. First, notice that BMO is not round (A) but inferiorly elongated to accommodate the passage of the CRA and CRV (blue vessel) to the retina. Notice also, that BMO is shifted inferiorly relative to the ASCO and PSCO. Second, notice the space between the neural tissue and the BMO, ASCO and PSCO in panel (B). At the level of BMO, the nerve abuts BM superiorly and there is space inferior to the nerve (filled by the CRV and CRA) as noted above. Note in Panel B, that because of the shift in BMO relative to the ASCO and PSCO, BM effectively extends beyond the scleral portion of the neural canal. Superiorly this creates an edge or extension of BM against which the neural bundle abuts. Inferiorly, it creates an elongation of BMO to accommodate the inferior vessels. The net effect of this shift is to create a superior-directed, oblique passage of the nerve through the wall of the rat eye that is best appreciated in Panels B, E and F. At the level of its passage through the anterior (ASCO) and posterior (PSCO) sclera the nerve is surrounded by a vascular plexus which we are not able to independently delineate using our current method. Third, there are three long posterior ciliary arteries which branch from the CRA behind the globe and pass together through the sclera (inferior to the neurovascular canal) to achieve the choroid. Their tangential rather than perpendicular passage through the sclera is depicted in (E) and (F) (see also Figure 4F, 4H and Figure 7). Fourth, short posterior ciliary arteries (defined to be primary branches of the ophthalmic artery that individually pass through the sclera) were not identified. Instead the few arteries that independently pass through the sclera superiorly (G), where identified, appear to be retrobulbar branches of the LPCAs. Fifth, unlike the primate eye in which there are 15 short posterior ciliary arteries that are relatively equidistant from one another around the circumference of the optic nerve, the choroidal blood supply in the rat appears to originate inferiorly from a dense plexus of intrascleral LPCA branches which effectively fill the space between the three LPCA branches (see Figure 1D – 1F). Some of these vessels extend within the sclera and choroid superiorly, around the neurovascular canal. It is the density of the LPCAs and their intrascleral branches inferiorly that suggests an “effective”second opening in the sclera. Sixth, the CRA and CRV are separated from each other and the nerve proper and supported by a “sling”(magenta) of scleral tissues that forms the inferior border of the neurovascular scleral canal opening and the superior border of the more-inferior arterial opening. N – Nasal; T – Temporal; I – Inferior; S – Superior.