672 THE EYE 



the arteries; they converge to form a single efferent vessel, the vena cen- 

 tralis retinae. The retinal veins are peculiar in that their walls contain 

 no muscle. The optic nerve is supplied with small branches from the 

 arteria and vena ceutralis retinae in their passage through its substance. 

 In the fetus a small branch, the hyaloid artery, apparently the 

 direct continuation of the arteria centralis retinae, passes forward through 

 the vitreous humor to the posterior surface of the lens, whence capillary 

 vessels pass around the margin of the lens and are connected with the 

 anterior ciliary vessels at the margin of the iris. The hyaloid artery 

 supplies blood to the fetal tunica vasculosa lentis for the nutrition of 

 the developing lens. Before birth these vessels disappear; the hyaloid 

 artery remains for a time as a delicate fibrous strand, occupying the 

 persistent canalis hyaloideus or canal of Stilling,, which lies almost in 

 the visual axis and extends from the papilla optica to the posterior 

 surface of the lens. The hyaloid canal (also called the canal of Cloquet) 

 establishes a channel between the aqueous humor and the lymphatic- 

 spaces of the retina. In adult life both the vitreous humor and the 

 crystalline lens are bloodless tissues. 



2. The short ciliary arteries, twelve to fifteen in number, enter 

 the globe of the eye in a circle (circle of Zinn) which surrounds the 

 optic nerve. They supply branches to the meningeal sheaths of the optic 

 nerve and to the sclera, their main stems penetrating this coat to enter 

 the choroid. Here they subdivide to form the plexus of arteries in the 

 lamina vasculosa from which the vessels of the choriocapillaris are 

 supplied. The capillaries of the last-named layer unite to form small 

 venous radicals which converge toward the equator of the eye, where they 

 unite in a whorl-like manner to form the four or five vence vorticosce, 

 which pass obliquely backward through the sclera, receiving additional 

 branches from this coat, and finally emerging from the eye to empty 

 into the ophthalmic vein. 



The vessels of the choroid communicate posteriorly with those of 

 the optic nerve, and anteriorly, by a free anastomosis, with those of 

 the ciliary processes. 



3. The long ciliary arteries, two in number, enter peripherally at 

 the circle of Zinn on either side of the optic nerve, and pass horizontally 

 forward upon the outer surface of the choroid to the ciliary muscle. 

 Near the base of the iris they divide, and by anastomosis with each 

 other and with the anterior ciliary arteries form a vascular circle, the 

 drculus major, about the base of the iris. 



Prom this circle recurrent branches supply the ciliary body and 



