ORGANS OF SPECIAL SENSE 



283 



The Optic Nerve. Owing to the relations established by the 

 choroid fissure, the floor of the optic stalk is continuous from the 

 first with the inner layer of the retina (Fig. 96 B), and it furnishes 

 the path along which the optic nerve grows. The axones of the 

 optic nerve originate, for the most part, from the retinal neuro- 

 blasts, composing the layer next to the cavity of the optic cup, 

 and their growth is thus centripetal. They are first formed in 

 the fundus part of the retina, and grow in the direction of the 



Afes /r 





Fig. 163. — Outlines of sections in the planes a, h, c, e, of 



Fig. 163. (After Bernd.) 



bl. v., Blood ves.sel. i. 1., Inner or retinal layer of the 

 optic cup. o. 1., Outer or pigment layer of the optic cup. 

 P. inv., Angle of invagination of the pecten. Other ab- 

 breviations as before. (Fig." 162.) 



optic stalk between the internal limiting membrane and the neu- 

 roblast layer (ganglion cell laA-er), thus forming a superficial layer 

 of axones; their formation begins on the fourth day, and there is 

 a period al)out the end of this day when axones are found in the 

 distal part of the optic stalk, next to the bulbus oculi, but not 

 in the proximal part, next to the brain. This observation affords 

 conclusive proof of the retinal origin of the fibers of the optic 



