EYES. 201 
and mesenchymatous cells enter the optic cup through the chorioid 
fissure before its closure. Some suppose that the vitreous body arises 
from a modification of these cells, some regard it as an exudate from the 
blood-vessels, and others think it a retinal secretion. The fact that 
the blood-vessels mentioned do not occur in birds is of interest in this 
connection. In mammals, when the chorioid fissure closes, the vessels 
appear to enter through the centre of the optic nerve (central retinal 
artery and vein—fig. 207). In the early stages the retinal artery 
an 
(Ao 004 ay 
nO 5 yo 
Fic. 207.—Diagrammatic section of half a mammalian eye. a+, anterior chambcr; 
ca, ciliary arteries; c, eyelash (cilium); cj, conjunctiva; co, cornea; cp, ciliary process; cr, 
central retinal artery and vein; cs, conjunctival sac; ct, chorioid tunic; d, dura of optic 
nerve; 7, iris; on, optic nerve; os, ora serrata; pc, posterior chamber; pe, pigmented epithel- 
ium; 7, retina; sc, sclera; tg, tarsal gland; vv, vorticose vein; cc, zonula zinii. 
divides inside the cup, one branch (hyaloid artery) going through the 
vitreous body to the neighborhood of the lens, the other being distrib- 
uted over the inner surface of the retina. Later the hyaloid artery 
disappears, while retinal arteries are rare except in mammals. 
The outer wall of the optic cup forms the pigmented epithelium 
of the eye, developing a large amount of black pigment which eventu- 
ally surrounds and isolates the rods and cones, so that each can be 
affected only by the light which falls directly upon it. As will readily be 
understood the side of the pigment layer away from the retina corre- 
sponds to the deeper surface of the skin and so comes into relation with 
the connective tissue. From this is developed the envelopes of the 
eye—tunica vasculosa, sclera, etc. 
Surrounding the retina and pigmented epithelium and extending 
forward over the lateral parts of the lens is the tunica vasculosa, 
in which two parts are recognized, the iris and the chorioid. The 
