8o 



MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



ganglion cells in their turn produce from these outer or free 

 surfaces axons which rapidly grow from the cells to the choroid 

 fissure (thus forming a layer of nerve fibres over the ganglion 

 cells), and thence, through the groove in the optic stalk, to the 

 brain. These axons form the optic nerve, 1 which, as will readily 



FIG. 86. Retinal elements, after Ramon yCajal. BC, bipolar cone cell; BR, 

 bipolar rod cell; C, cone; CF, centrifugal fibre; G, ganglion cell layer; H, hori- 

 zontal cell; 76", inner granular layer ; IM, inner molecular layer; N, nerve fibres ; 

 OG, outer granular layer ; OAf, outer molecular layer; /v', rods; S, supporting cells. 



be understood, appears, after the closure of the choroid fissure, 

 as if it left the eye through the centre of the retina. As all 

 sense cells are lacking at the point of exit of the optic nerve, 

 this region forms the l blind spot ' described in all physiological 

 text-books. 



1 It was formerly thought that the optic nerve arose by a modification of the cells of 

 the optic stalk. Later, nerve fibres were described as growing from the brain to the eye ; but 

 while some fibres may arise in this way, the majority arise as described above. So far as 

 method of nerve formation is concerned, the optic nerve resembles the dorsal root of a spinal 

 nerve (pp. 47 and 60, foot-note.) 



