356 Journal of Comparative Neurology. 



same fluids as shown in Table i. Measurements were made as 

 nearly as possible along the axis of the eye. 



From this tabulation we see that the hardening fluids have 

 affected the retina very much as they did the whole eye. We 

 also perceive that there is a very great individual variation. 

 For example, the eyes which were hardened in Perenyi's fluid 

 show a thickness of retina ranging from . 192 mm. to .396 mm. 

 A very slight difference in the arrangement of the retina will 

 greatly alter its thickness. To illustrate: Figures 13 and 14 

 are specimens numbered 25 and 27 respectively and were hard- 

 ened in Perenyi's fluid. As shown in Table III, the former 

 has a retinal thickness of . 192 mm. and the latter .300 mm. In 

 each of these cases the distance along the axis of vision from 

 the posterior surface of the lens to the pigment layer is prac- 

 tically the same. In Figure 14 this whole distance is occupied 

 by the retina, while in Figure 13 the retina occupies only a 

 part. Such slight variations in the arrangement of the retina 

 may thus cause a great difference in its thickness. From 

 Table III we see that the average thickness of the retina of the 

 adult is .2884 mm. This is much thicker than Kohl found 

 the retina of Talpa europaea. He found the retina of the 

 adult .1313 mm. thick. The ratio of the thickness of the 

 retina to the axial diameter of the eye he represents as i : 7.49. 

 In the American mole the ratio is 1:2.85. The retina of the 

 European mole is not so crowded and lies in almost a normal 

 manner. 



The rod and cone layer is quite uniform. It has an aver- 

 age thickness of .0075 mm. This does not represent the 

 length of these elements. Owing to the processes of the pig- 

 ment cells more or less concealing the rods and cones, it is 

 almost impossible to measure to their external ends. A part of 

 the internal segments is all that is to be noticed. 



When the rods and cones are isolated they present char- 

 acteristics very similar to those of the normal mammalian eye. 

 The rods are quite slender and elongated. They have a total 

 average length of .0286 mm. The internal and external seg- 

 ments are readily distinguished (Fig. 30, r). The former has 



