LYMPHATIC SYSTEM OF EYEBALL 1065 



where they form a plexus, from which proceed branches for the ciliary muscle, the 

 iris, and the cornea. 



The nerves of the iris enter it at its cihary border, and run toward its pupillary edge, losing 

 their medullary sheath sooner or later, and supplying especially the sphincter muscle. The 

 corneal nerves form an annular plexus near the limbus, from which a few twigs proceed to the 

 sclera and conjunctiva, while most of the offsets enter and run radially in the corneal stroma, 

 branching and anastomosing so as to form a plexus. The nerves entering the cornea are about 

 sixty in number, each containing from two to twelve non-meduUated nerve-fibres. 



Blood-vessels of the eyeball. — ^The eyeball receives blood from two sets of 

 vessels, viz., the retinal and the ciliary arteries, as described in the section on 

 the Blood-vascular System. 



1. The arteria centralis retinae either comes direct from the ophthalmic artery, or from one 

 of its branches near the apex of the orbit. Entering the optic nerve twenty millimetres or less 

 behind the globe, it runs forward in its axis to the end of the nerve-trunk, and then divides into 

 branches which run in the inner layers of the retina, and divide dichotomously as they radiate 

 toward the equator. The smaller branches lie more deeply in the retina, but none penetrate into 

 the nerve-epithelium, so that the fovea centralis is non-vascular. In the retina, the branches 

 of the central artery do not communicate with any other arteries, but while still in the optic 

 nerve fine communications take place between this artery and neighbouring vessels. Thus 

 (a) minute twigs from it, which help to nourish the axial part of the nerve, communicate with 

 those running in the septa derived from the pial sheath. Again, as the nerve pas.ses through 

 the sclera, it is surrounded by a vascular ring [circulus vasculosus n. optici (Halleri)], formed 

 of fine branches derived from the short posterior ciliary arteries; fine twigs passing inward 

 from this ring to the optic nerve join the ve.ssels of the pial sheath, and (6) an indirect communi- 

 cation is thus brought about between the retinal and ciliary vessels. Finally, as the nerve passes 

 through the chorioid, there is (c) a direct connection between these two sets of vessels, the cap- 

 illary network of the optic nerve being here continuous with the chorio-capillaris. Not infre- 

 quently, a branch from a short posterior ciliary artery pierces the optic papilla, and then courses 

 over the adjoining retina (a cilio-retinal artery), supplying the latter in part in place of the cen- 

 tral artery. 



The branches of the a. centralis retinae in the retina are : arteriola temporalis retina; supe- 

 rior, arteriola temporalis retinse inferior, arteriola nasalis retinae superior, arteriola nasalis 

 retinae inferior, arteriola macularis superior, arteriola macularis inferior, arteriola retinae 

 medialis. 



The vena centralis retinae returns the blood of the corresponding artery and has branches 

 corresponding to those of the artery. 



2. The ciliary system of blood-vessels (see Blood-Vascular System). — There are three sets 

 of arteries belonging to this system, all derived directly or indirectly from the ophthalmic 

 artery. 



(1) Short posterior ciliary arteries twelve to twenty in number, pierce the sclera round the 

 optic nerve entrance, and are distributed in the chorioid. Before entering the eyeball, small 

 twigs are given off to the adjoining sclera and to the dural sheath of the optic nerve. 



(2) Two long posterior ciliary arteries, medial and lateral, piercing the sclera further from 

 the nerve than the short ciliaries, run horizontally forward between the sclera and chorioid, 

 one on each side of the globe. On arriving at the ciliary body, they join with the anterior ciliary 

 arteries, forming the circulus arteriosus major, which sends off branches to the ciliarj^ processes 

 and the iris. The long ciliaries also give twigs to the ciliary muscle, and small recurrent branches 

 run backward to anastomose with the short ciliary arteries. The arteries of the iris run radially 

 toward the pupillary border, anastomosing with one another opposite the outer border of the 

 sphincter and forming there the circulus arteriosus minor. 



(3) The anterior ciliary arteries come from the arteries of the four recti muscles, one or 

 two from each; they run forward, branching as they go, and finally pierce the sclera near the 

 corneal border. Externally to the globe they send twigs to the adjoining sclera, to the conjunc- 

 tiva, and to the border of the cornea. After passing through the sclera the arteries enter the 

 ciliary muscle, where they end in twigs to the muscle and to the circulus arteriosus major, and 

 in recurrent branches to the chorioid. 



Veins. — The venous blood from almost the whole middle coat (chorioid, ciliary processes 

 and iris, and part of the ciliary muscle) ultimately leaves the eyeball by — (1) the venae vorticosae, 

 which have been already noticed in describing an antero-posterior section through the globe. 

 One large vein passes backward from each vortex, piercing the sclera obliquely; it is joined by 

 small episcleral veins when outside the globe. 



(2) The anterior ciliary veins commence by the junction of a few small veins of the ciliary 

 muscle; they pass outward through the sclera near the corneal border, receiving blood from the 

 veins in connection with the sinus venosus of the sclera, and afterward from episcleral and con- 

 junctival veins, and from the marginal corneal plexus. Finally they join the veins running 

 in the recti muscles. 



Lymphatic system of the eyeball. — Apart from those in the conjunctiva there 

 are no lymphatic vessels in the eyeball, but the fluid is contained in spaces of vari- 

 ous sizes. These are usually divided into an anterior and a posterior set. 



1. Anteriorly, we have the anterior and posterior aqueous chambers (together 

 composing the aqueous chamber of the eye), which communicate freeh' through the 

 pupil. The aqueous humour is formed in the posterior of these chambers by 



