MAMMALS. 



EYES OF THE COMMON MOLE. 



Dr. J. R. Slonaker has found that the eye of the mole (Scalops aquaticus 

 machrinus) lies embedded in the muscle beneath the skin, where it appears as 

 an inconspicuous dark spot. It is situated well forward on the side of the snout. 

 The eye is degenerate and is no longer capable of functioning in distinct vision. 

 The most noticeable changes which have occurred are: 



1. The great reduction in the size of the eye. 



2. The much crowded condition of the retina as a result of the decrease in size 

 of the eye as a whole. 



3. The noticeable reduction in the size, or the complete absence, of the aqueous 

 and vitreous chambers. 



4. The varied modification of the shape and size of the lens, also the peculiar 

 cell structure of the lens. 



All the structures of the normal mammalian eye are present in some form or 

 other, (i) The conditions found in the adult and at birth have been studied. Very 

 little difference is seen in these two stages excepting an increase in size. 



The eye muscles and the optic nerve are easily traced back to the skull. At 

 birth the nerve presents in its course from the eye to the skull a peculiar arrange- 

 ment. The course is marked by numerous cells and few or no fibers. At the eye 

 there is a rapid change from this cell condition to the fiber condition of the nerve 

 tract. The fibers have not apparently grown much beyond the limits of the eye. 

 In the adult the fibers can be traced to the skull. 



The eye cleft is very small and of practically the same diameter in both hori- 

 zontal and vertical sections through it. It meets'the eye at such an angle that it 

 is impossible for rays of light, should any enter, to'pass through the eye along the 

 axis of vision. 



All the elements of the normal retina are present, but, owing to the much 

 crowded condition, the ganglion-cell layer is much increased in thickness. 



The lens, which is found in a great variety'of shapes and sizes, is composed 

 of peculiar cartilage-like cells with well-defined 'nuclei. It is therefore incapable 

 of functioning as a normal lens. 



It is very doubtful, therefore, whether the eye of the mole functions in any 

 sense. At best it can do no more than distinguish between light and darkness. 



25 



