108 BLIND VERTEBRATES AND THEIR EYES. 



layer consists normally of two sublayers ; the outer one has both nerve fibers and 

 nerve cells — the latter according to Krause being the terminal stations of the optic 

 nerve — and the inner sublayer has the terminal stations of the fifth layer in it. 

 The outer sublayer is entirely atrophied in the lobes of the blind fish ; and the inner 

 one, if at all present, is indistinguishable from the third layer (3 and 4 in fig. 34 b). 



The fifth layer is reduced to diagonal fibers. The descending fibers which join 

 the optic tracts are atrophied. The diagonal fibers are more apparent than in the 

 normal brain. These fibers form a broad commissure in the torus longitudinalis, 

 which runs laterally to the outer edge of the lobes, where it turns back into the 

 substance of the brain just beneath the ventricle and becomes diagonal. Cross- 

 sections of fibers arising from various levels of the lobes are shown (5 in fig. 34 b). 



The sixth layer is a granular layer. Its thickness is less than in the normal 

 brain. No other change is noticeable. The thickness of the seventh layer, epen- 

 dyma, is not more than half that of a normal brain. The cells show some shrinkage. 



The differences in the lobes thus appear to be: first, in the atrophy of the 

 second layer; second, the outer sublayer of the fourth layer is entirely gone; 

 third, the descending fibers of the fifth layer are wholly wanting; fourth, the granu- 

 lar layer is not so thick and the ependyma is not only thinner but reduced in the 

 numebr of its cells. 



The optic tracts, that part of the nervous tissue which lies between the optic 

 lobes and the optic chiasma, are entirely wanting. The space occupied by these 

 tracts in the normal brain is in this brain partially occupied by tissue in which I 

 have not been able to make out any structure. All the stains that have been tried 

 have failed to reveal any cells. These tracts do not take the stains with the same 

 readiness and in the same degree that those in normal brains do when subjected to 

 exactly the same treatment. Three fishes, Amblyopsis, Campostoma, and Eupomotis, 

 were killed and the heads placed in Fohl's mixture for the same duration of time. 

 The brains were removed from the skull as soon as they were sufficiently hardened 

 and were placed in the same bottle in order that the conditions might be alike. 

 The three were embedded in the same block and sectioned side by side. The 

 tissue of the tracts of the brains of Campostoma and Eupomotis differentiated very 

 well — but the degenerate brain showed no structure. 



In the dissections of the head of the blind fish, I have been unable to find any 

 indications of optic nerves leaving the lobes. In both the dissections and the 

 sections which have been made of the entire head and brain, there seems to be no 

 break in the enveloping membranes on the anterior ventral surface of the lobes 

 where the optic nerves originate. The vestiges of the optic nerve can be followed 

 backward from the eye for a short distance. The only tracts leading away from 

 the lobes are those which connect them with cerebral hemispheres and cerebellum. 

 Those which pass forward to the hemispheres are from the diagonal fibers of the 

 fifth layer. These pass laterally, but before reaching the lateral aspect of the 

 lobes, turn downward through the granular and epithelial layers, and then course 

 forward toward the ventral surface of the hemispheres. 



