Ophthalmic and CRAs. A, Superior view of the right orbit. The levator, superior rectus, and superior oblique muscles have been reflected to expose the ophthalmic artery coursing above the optic nerve. The CRA is not seen in this view because it arises below the optic nerve. The ophthalmic artery passes above the optic nerve and between the superior oblique and medial rectus muscles where it gives rise to the anterior and posterior ethmoidal arteries. The ophthalmic artery was divided into three segments based on the two curves of the ophthalmic artery relative to the optic nerve. The first or proximal segment extends from the orbital apex and along the inferolateral aspect of the optic sheath. The second segment (blue bracket) begins where the artery curves medially to pass above or below the optic sheath and ends at the point at which it turns forward on the medial side of the sheath. The third segment (green bracket) extends forward on the medial side of the optic nerve where it follows a tortuous course. The second and third segments are exposed here. B, The middle and distal segments of the ophthalmic artery and optic nerve have been removed to expose the proximal segment of the ophthalmic artery (yellow bracket) and the origin of the CRA. The CRA arises as the first branch of the ophthalmic artery in a common stem with the posterior ciliary artery. The CRA enters the lower surface of the optic nerve, after which it passes to the center of the nerve. C, Superolateral view of another nerve with the levator, superior and lateral muscles, and a portion of the optic nerve removed. The ophthalmic artery can be followed forward from the internal carotid artery. The CRA arises from a common trunk with the posterior ciliary artery. The CRA coursed below the posterior two-thirds of the length of the intraorbital part of the optic nerve and penetrates the anterior third of the optic sheath. The distal part of the ophthalmic artery has been removed. D, Enlarged view of the site of penetration of the optic sheath. The CRA penetrates and courses forward a short distance in the sheath before passing to the center of the nerve. E, Anterior view of the right optic nerve after removal of the globe. The CRA courses in the center of the nerve. (Images courtesy of AL Rhoton, Jr.)