THE EYE. 389 



doubling up is not confined to the optic cup itself, but extends 

 a certain distance along the optic stalk towards the brain. The 

 consequence is that the part of the optic stalk near to the 

 eye (Fig. 155) is not a simple tube with thick walls, but a 

 tube deeply grooved along its under surface by folding of its 

 walls. 



This groove, being continuous with the choroidal fissure, 

 leads into the cavity of the optic cup, i.e. of the globe of 

 the eye ; and it is by running along this groove that the 

 central artery of the retina, a branch of the internal carotid 

 artery (Fig. 155, ao), gains admittance to the interior of the 

 eye. This artery supplies the retina throughout life, and during 

 the development of the eye supplies the vitreous body and the 

 capsule of the lens as well. 



Of the two walls of the optic cup, the distal or inner wall 

 (Fig. 155. oc) is, from the first, much thicker than the proximal 

 or outer wall, OD ; the difference being a very pronounced one by 

 the fourteenth day. From the inner layer, oc, the entire thick- 

 ness of the retina proper is developed, the rods and cones being 

 processes from its outer surface, which do not appear until shortly 

 before birth. 



The outer and thinner wall of the optic cup, OD, becomes con- 

 verted, as in Vertebrates generally, into the pigment layer of 

 the retina ; a stratum of hexagonal cells, closely fitted together, 

 with which the retinal rods ultimately acquire very close rela- 

 tions. In the cells of this layer, granules of pigment are deposited 

 at an early stage ; and up to a late period of development the 

 black colour of the eye is due to this layer, the choroid coat of 

 the eye not developing, or acquiring pigment, until very near the 

 time of birth (c/. Fig. 156). 



Near the free edge of the optic cup, the two layers are ot 

 approximately equal thickness (Fig. 156), and grow forwards 

 in front of the lens to form the pigmented epithelium of the 

 posterior surface of the iris. 



The optic nerve. It is at present uncertain whether the 

 fibres of the optic nerve of the rabbit are developed in situ, 

 from the walls of the tubular optic stalk, or whether, as seems 

 far more probable, they arise in the retina and grow inwards 

 along the optic stalk to the brain. The nerve fibres from the 



