Horizontal sections in a fetus with a CRL of 272 mm (normal fetus). (A) The most superior site. (G) The most inferior site. (A and B) The origin of the SR surrounds the optic nerve exit. (B) The accessory head of the MR (MRacc) originates from the sphenoid near the SR origin. (D) It provides an independent muscle bundle. (E) The main head of the MR is distant from and inferior to the accessory head. (G) The MR and LR sandwich the IR in the common tendon. ABN, abducent nerve; ICA, internal carotid artery; OCN, oculomotor nerve; ON, optic nerve; OPA, ophthalmic artery; OPN, ophthalmic nerve.