106 



BLIND VERTEBRATES AND THEIR EYES. 



THE BRAIN OF AMBLYOPSIS. 

 (By E. E. Ramsey.) 



A comparison of the microscopic appearances of the brain of a normal fish 

 and that of the blind fish, Amblyopsis spelceus De Kay, discloses a number of inter- 

 esting conditions. The optic lobes and the optic tracts are measurably degenerate. 

 The hemispheres are larger in Amblyopsis than in the average of normal brains. 

 The brains of Campostoma anomalum, Percina caprodes, Eupomotis gibbosus, and 

 Amblyopsis were measured with regard to the comparative widths of the optic 

 lobes and the hemispheres. Five fishes of the same length were taken of each 

 species. The averages obtained are as follows : 



It is thus seen that the hemispheres are relatively larger in the blind fish than in 

 the more normal forms, and that the optic lobes are relatively much smaller in the 

 former. 



There is no noticeable variation in the cerebellum. In length there is a marked 

 shrinkage, chiefly in the optic lobes, as shown by the position of the cerebellum 

 which lies directly on the lobes. In the normal brain the cerebellum is situated 

 well back, hardly reaching the lobes. The following table gives an idea of the 

 length of the brain, as compared with the length of the fish. The brain length is 

 measured from the tip of the olfactory lobes to the posterior part of the cerebellum : 



The result shows the brain of Amblyopsis to be only two-thirds as long as that 

 of Campostoma. This shrinkage in width and length is great enough to show itself 

 in the extent to which the cranial cavity is filled. A great depth of fatty tissue cov- 

 ers the dorsal surface of the brain. The only other external modification of any 

 note is the absence of either optic nerves or optic chiasma. 



The optic lobes are normally composed of 7 layers, which from outside to inside 

 are as follows : 



(1) A peripheral zone. 



(2) An optic fiber layer from the optic nerve. 



