All the authors who investigated the anatomical features of the CRA described the ophthalmic artery, a branch of the internal carotid artery, as the source of origin of the CRA. In my study [11, 12], the CRA was a branch of the ophthalmic artery, irrespective of whether the latter was a branch of the internal carotid artery (Figs. 2.1b–e, 2.2, 2.3, and 2.4) or, rarely, of the middle meningeal artery (Figs. 2.1f, g, 2.5, and 2.6). Of six specimens, where the main source of blood supply to the ophthalmic artery was the middle meningeal artery, in four there was also a small trunk connecting the internal carotid artery to the main ophthalmic artery (Figs. 2.1b–e, 2.5, and 2.6), and in the other two the connection between the initial part of the ophthalmic artery and the internal carotid artery was absent (Fig. 2.1f, g), so that the blood supply to the CRA was from the middle meningeal artery. Morandi et al. [14] described a case where the ophthalmic artery arose from the middle meningeal artery and was associated with CRAO, and they considered that such an unusual origin could be a factor for CRAO. My study did not bear out that speculation.

In my study [11, 12], two specimens had two CRAs, each arising independently from the ophthalmic artery (Fig. 2.2). That had not been previously reported. In one of these specimens, the two CRAs pierced the sheath of the optic nerve 12 mm and 5 mm behind the eyeball on its lower lateral aspect and ran independently up to the optic disc. At the optic disc the two joined to form a loop, from the summit of which arose terminal branches which followed the usual course (Fig. 2.2). In the other specimen, the two arteries united to form one trunk at the point of entry into the sheath of the optic nerve.

According to Meyer [1] and Kershner [4], the CRA branches from the ophthalmic artery, while the latter is still within the sheath of the optic nerve at the apex of the orbit. Sudakevitch [5] reported that the artery may arise anywhere along the first part of the ophthalmic artery and very rarely distal to this point.

The order and site of origin of CRA from the ophthalmic artery in my study [11, 12] is summarized in Table 2.1. The CRA was described by Tsutsumi and Rhoton [15] as the first branch of the ophthalmic artery in 67 % and the second branch in 28 % and by Kocabiyik et al. [16] the third branch in 5 % and the first branch in 93.3 %.

This was described differently by various authors. The CRA has been described as arising as an independent branch [4, 9, 15] and in common with the posterior ciliary artery [2, 4, 15], medial posterior ciliary artery [1, 2, 5, 9], and lateral posterior ciliary artery [9]. Modes of origin of CRA from the ophthalmic artery in my study [11, 12] are summarized in Table 2.1. In Fig. 2.3, the CRA arises from the ophthalmic artery in common with the lateral posterior ciliary artery and in Figs. 2.4 and 2.5 in common with the medial posterior ciliary artery.

This is from its origin from the ophthalmic artery till it pierces the optic nerve sheath. In my study [11, 12], in most specimens the CRA had a tortuous intraorbital course (Figs. 2.2, 2.3, 2.4, 2.5, 2.6, and 2.7). This tortuosity was particularly marked near the point of entry into the dural sheath, and in this locality the coils of the artery are usually fixed to each other by thin fibrous sheets. In 90 % of specimens this part lay free in fatty areolar tissue around the optic nerve, lightly adherent to the sheath of the nerve, and in 6 % embedded in the folds of the dura beneath the optic nerve. In 4 % it was adherent only near the site of entry into the sheath for a millimeter or so and was free more proximally. Sometimes the CRA is surrounded in its intraorbital course by a thick plexus of veins derived from the dural sheath and central retinal vein. Tsutsumi and Rhoton [15] stated that the diameter of the CRA at its origin from the ophthalmic artery averaged 0.4 mm (range, 0.3–0.6 mm). According to Kocabiyik et al. [16], the mean diameter of the CRA was 0.47 ± 0.05 mm on the right side and 0.46 ± 0.04 mm on the left side.

This part lies between the points where the CRA has pierced the dural sheath of the optic nerve and the nerve itself. In my series [11, 12], the existence of distinct subarachnoid and subdural spaces was clearly seen in the reconstructions of serial sections (Figs. 2.9c, d). In its intravaginal course, the artery usually runs forward for a short distance in the subdural space (Fig. 2.9c) and for a much greater distance in the subarachnoid space (Figs. 2.9d, 2.10, 2.11, and 2.12). The artery carries a prolongation from the dura, and it also carries from the arachnoid a tubelike prolongation, which surrounds it in the subarachnoid space. The intravaginal part of the artery is usually 0.9–2.5 mm long. The artery may be accompanied by the central retinal vein; although this was described as invariable by some [3, 4], it was found in only 8 out of the 14 of my specimens [11, 12] in which the vein could be studied. In 8 % of the specimens, the CRA formed a tortuous loop in its intravaginal course (Fig. 2.12), and in such cases the intravaginal part was much longer than usual. In 1 % the artery looped and ran backward.

This lies in the optic nerve. In my study [11, 12], the artery entered the optic nerve in a well-defined fissure on the inferior surface of the optic nerve anterior to the site of penetration into the dural sheath (Figs. 2.13, 2.14, and 2.15) and carried a fold of pia with it (Fig. 2.16). The site of entry into the nerve was always 1.2–3.5 mm anterior to the site of penetration into the dural sheath (Figs. 2.8, 2.10, 2.11, and 2.12), except in one specimen in which it was situated 4 mm posteriorly. The intraneural part of the CRA has two distinct sections – vertical and horizontal (Figs. 2.8 and 2.17). In my study [11, 12], the initial vertical part reached the center of the nerve by running upward and slightly forward in 89 % (Figs. 2.8 and 2.17), vertically upward in 8 %, and upward and backward in 3 %. The transition from the vertical to the horizontal parts was gradual, no sharp angle being formed at this point. In the specimens with two trunks of the CRA, the vertical part was missing, and the two trunks ran straight to the optic disc from their point of penetration into the nerve (Fig. 2.2). The horizontal part runs forward horizontally in the center of the optic nerve and lamina cribrosa to reach the optic disc, where it divides into its terminal branches (Fig. 2.8). Magitot [2] and Beauvieux and Ristitch [3] reported that the artery was surrounded in this part by vascular fascia which gradually diminished and came to an end just at the point of entry of the artery into the eyeball. In my serial sectioning study [11, 12], the artery was enclosed in a fibrous tissue envelope, which in turn was continuous with the fibrous septa of the optic nerve (Fig 2.18). The artery was loose inside this envelope, the surrounding space being visible on dissection (Fig. 2.18). No such space was seen around the central retinal vein, which lay beside the artery on the lateral side and was also enclosed in a fibrous envelope. The two vessels may lie in close apposition (Figs. 2.18a, b), or they may be separated by fibrous tissue (Figs. 2.18c, d), and at places even nervous tissue may intervene. I found that it was not essential for the CRA to travel in the center of the optic nerve to reach the lamina cribrosa. In one specimen the vertical section was virtually absent, so that the artery ran straight to the center of the lamina cribrosa from its point of penetration into the nerve; the central retinal vein, however, occupied the usual position at the center.