257 
At a glance one can see that there is very little difference excepting 
in the thickness of the retina, that of the cave rat being thicker. This 
difference, however, may be due to the fact that Fig. 5a, is from a very 
large cave rat, while Fig. 5b is from a half-grown gray rat. The thick- 
ness, however, bears about the same ratio to the size of the eye in each 
I 
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PA 
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Fig.5. Semi-diagrammatic camera drawings (* 265). 
Wi 
Yi 
I 
a. Mammoth Cave Rat. 
b. Common Gray Rat. 
1. Nerve Fibre Layer. 
2. Nerve Cell Layer. 
Inner Molecular Layer. 
Inner Nuclear Layer. 
Outer Molecular Layer. 
Outer Nuclear Layer. 
ano e Ww 
Ci. Cet Ge Siem) 
Rod and Cone Layer. 
Pigment Layer. 
Oo oo 
. . 
Supporting Fibres of Miiller. 
case. This greater thickness is largely due to an increase in the size of 
the cells of corresponding layers of the retina in the cave rat. Only a 
single instance need be given. The rod and cone layer of the cave rat is 
composed of decidedly longer and larger elements than the same layer 
of the common rat. But with the exceptions of these minor differences 
in the thickness of the layers and in the size of the cells, the two retinae 
are nearly alike. 
Basing our conclusions on the histological structure of the eye, we 
may infer that the cave rat has the power of seeing as distinctly as the 
common gray rat. 
17—ScIENCE. 
