BLOOD VESSELS OF THE EYE 599 



with small branches from the arteria and vena centralis retinae in 

 their passage through its substance. 



In the fetus a small branch, 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. Before birth these 

 vessels are occluded, their investing fibres remaining for a time as 

 a minute fibrous canal, the 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. 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 chorio-capillaris 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 at the circle 

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

 ward 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 circulus major, about the base of the iris. 



From this circle recurrent branches supply the ciliary body and 

 anastomose with the vessels of the choroid ; other branches pass 

 into the iris and, converging toward the visual axis form, just out- 

 side the pupillary margin, a second circle of anastomosis, the cir- 

 culus minor. 



The veins of the iris and ciliary body follow closely the dis- 

 tribution of the arteries, the greater portion of their blood return- 



