NUTRITION OF THE RETINA 51 



tion is wholly devoted to the nutrition of the visual cells. The turnover 

 of substances must be very great, for the chorioid is very rich in blood 

 vessels which indeed comprise most of its bulk in many animals. 



Just outside of the lamina vitrea lies a network composed of broad, 

 flat capillaries. This 'choriocapillaris' reticulum (Fig. 6b, p. 14) is so fine- 

 meshed that its capillaries total a greater portion of its area than do 

 the spaces between them. It is with the blood in the choriocapillaris that 

 the visual cells make their exchanges of supplies and wastes, liaison 

 being effected by the pigment epithelium which is thus taking in and 

 giving off materials at both of its surfaces continuously. The retina 

 often has blood vessels clinging to or embedded in its inner surface, but 

 these are usually concerned only with the nutrition of the inner layers 

 of the retina. Even where (as in most mammals) capillary branches of 

 these vessels invade the retina itself, they almost never reach outward 

 beyond the inner nuclear layer and obviously belong only to the vitread 

 portion of the retina. Such a capillary shows in Figure 19. 



The choriocapillaris is supplied with blood by a layer of arteries 

 outside of it in the chorioid, and drains into a layer of large inter-con- 

 necting veins which lie on the scleral side of these arteries (Figs. 4a, 6a; 

 pp. 8, 14). The veins converge in the four quadrants of the eyeball to 

 pour their contents into the four great 'vorticose veins' which conduct 

 the blood away from the equator of the eye. Other vessels also penetrate 

 the sclera anteriorly and supply or drain structures other than the retina. 

 The vessels mentioned above, which supply the inner layers of the retina, 

 are few and are branches of vessels which enter the eyeball in or along 

 with the optic nerve. True retinal vessels are present only in the eels and 

 the mammals — -and not even in all of the latter, some of whose retinas 

 (e.g., in the rhinoceros) are as completely avascular as those of the lower 

 vertebrates. 



All of the vessels concerned with the eye apart from the retina— and 

 even including those last mentioned above — do not, taken together, com- 

 pare in abundance with the rich chorioidal circulation. This latter exists 

 solely for the benefit of those cells of the whole eye which are most 

 important, if any are that: the rod and cone visual cells. 



The Optic Nerve — The human optic nerve takes a long, slightly 

 undulant course to the apex of the orbit and there enters the cranium 

 (Fig. 16, p. 37). It is flexible, and by its length allows enough slack to let 

 the eye rotate freely. It contains more than a million nerve fibers, most of 



