The order in which these arteries arise from the ophthalmic presents many varieties; but they are constant in their distribution.
1. The lachrymal artery is one of the largest branches of the ophthalmic : it sometimes comes from the middle meningeal, and enters the orbit by the foramen lacerum orbitale of the sphe noid bone. It runs forwards between the ex ternal wall of the orbit and the rectus externus, giving branches to that muscle, the periosteum, levator palpebrm superioris and sheath of the optic nerve. One of its branches traverses the mita bone, and entering the temporal fossa anastomoses with the anterior deep temporal ; another little branch frequently traversing this bone passes outwards through the same hole with the itervus subcutaneus maize, and anas tomoses with branches of the transversalis faciei. The continuation of the artery then divides into several branches which are distributed to the lachrymal gland and the external part of the upper eyelid, anastomosing with the palpebral and the temporal arteries.
2. The central artery of the retina (arteria centralis realm) penetrates the substance of the optic nerve to enter a canal in its centre, the porus opticus, in which it passes forwards, and is distributed to the retina, the vascular layer of which it forms by its mmifications.
3. The supra-orbital arises after the centralis retina2, passes forwards along the superior wall of the orbit above the levator palpebrx supe rioris and superior rectus, giving branches to these muscles, the periosteum, and the scle rotic : on reaching the margin of the orbit, it passes out through the superciliary foramen, along with the frontal branch of the ophthalmia nerve, giving in its passage a, branch which enters the substance of the frontal bone; this artery then mounts beneath the corrugator su percilii and orbicularis palpebrarum muscles; and is expended on these muscles, the occipito frontalis and the integuments; it anastomoses with branches of the lachrymal and frontal.
4. The ciliary arteries sometimes amount in number to thirty or forty; they consist of three sets : the posterior or short, the long, and the anterior. The posterior ciliary arteries are very numerous, sometimes amounting in number to thirty or forty : although mostly arising from the ophthalmic, some of them come from the inferior muscular, the supra-orbital, posterior ethmoidal or lachrymal ; they run along the optic nerve very tortuous, and entangled with the ciliary nerves, anastomosing freely with each other.
The posterior or short ciliary arteries pierce the sclerotic close to the entrance of the optic nerve; some of their branches are distributed to that membrane in which they anastomose with branches from the muscular arteries ; while all the others advance nearly parallel, dividing at very acute angles into numerous smaller twigs ; these branches are at first ex ternal to the choroid ; but in their course for wards they penetrate to the internal surface of that membrane, and becoming more numerous from having undergone new subdivisions, forni a network of anastomoses from which several branches are sent to the ciliary margin of the iris, where they anastomose with the anterior ciliary, but a greater number are given to the ciliary processes in the centre of which they form a very fine network, a,nd finally end in a circle of anastomoses surrounding the margin of the circle in which these processes terminate internally.
The long ciliary arteries are two in number, one internal, the other external ; they are larger than the short ciliary arteries among which they arise, but pierce the sclerotic obliquely at a greater distance from the optic nerve ; they pass forwards between the sclerotic and cho roid, and having arrived at the ciliary ligament, they divide each into two long branches which sepamte from each other at obtuse angles, and, coursing along the ciliary margin of the iris; form a circle around the greater circumference of that membrane which receives branches of anastomosis from the short ciliary arteries. From the interior of this circle numerous branches arise, each of which divides into two, which diverge at obtuse angles, and, anastomo-: sing with each other and with the anterior ciliary, form another arterial circle within the former. Thus there are two arterial circles, one within the other at the greater circumference of the iris. From the concavity of this inner circle the arteries of the iris arise. These arte ries are very numerous ; they converge in ser pentine lines towards the papillary margin of the iris, where they anastotnose, in the manner of the mesenteric arteries, to form the lesser arterial circle of the iris. All these arteries, however, do not contribute to form this lesser arterial circle ; a great number pass beyond it, and, along with the branches which arise from its concavity, advance towards the pupil. There are thus three arterial circles in the iris, two close together at its greater circumference or ciliary margin ; the third much smaller, sur rounding its pupillary margin, and commu nicating with the preceding by a radiation- of branches situated on the anterior surface of the iris.