SECT. 226. J 



IN THE RETINA. 



5S9 



:m 





darker than previously. Fibres so treated swell up again in acetic 

 acid, and are dissolved by alkalies; so that perhaps they consist 

 chief! v of a nitrogenous substance. 



With regard to the course of the nerve-fibres in the retina, 

 this much is certain, that they radiate in all directions from the 

 colliculus nervi optici, form a connected membranous expansion, 

 which extends as far as the ora serrata, and present no interrup- 

 tion of any size except in the region of the yellow spot. In this 

 proper nervous tunic, the nerve-fibres are collected together to 

 found larger and smaller bundles, slightly compressed laterally, 

 and measuring o*oi" to 00 12'" broad, which either anastomose 

 with each other at very acute angles, or run for long distances 

 parallel to each other. At the yellow spot, only a small part of 

 the optic fibres proceed directly to Fig. 230. 



its inner end ; much the larger por- 

 tion of the fibres which are destined 

 for the lateral parts of the spot, 

 describe a series of curves, which 

 take larger and larger sweeps as 

 they are directed farther forwards. 

 At the yellow spot itself, all these 

 fibres lose themselves in its deeoer 

 portion, among the nerve-cells, so 

 that there is here no superficial layer 

 of optic fibres; the nerve-fibres of 

 this spot most probably arise from 

 the processes of its cells (Remak). 

 External to the yellow spot, the 

 fibres gradually become more spread 

 out, and at last re-assume a straight 

 course. As for the terminations 

 of these nerves, the most recent 

 investigations have rendered it more 

 than probable that they all pass 

 into the processes of the nerve-cells 

 of the retina, a condition which, 

 histologically speaking, would be better designated by saying that 

 they all take their origin from these cells. The thickness of the 

 layer of optic fibres measures crogo'", close to the entrance of the 

 nerves ; anteriorly, four to six lines from the colliculus, o - 028"' to 

 o , 036" / ; quite in front, and at the border of the macula lutea, 

 about o - oo2"'; at the bottom of the eye, 0-036'" ; two lines ex- 



t 



■''.'ilk, 





Distribution of the fibres at the en- 

 trance of the optic nerve and round the 

 yellow spot, a, entrance of the optic 

 nerve; 6. yellow spot ; c. arcuated fibres 

 on both sides of it ; d. arcuated fibres on 

 its outer side ; ee. radiating fibres of the 

 optic nerve. 



