I08 HODGE. [Vol. VII. 



This also may be true. Is it the case of a cell doing its work, 

 and, under the hail of changes which the external world showers 

 upon it, dying of fatigue .-* And this may be true. So that we 

 have epitomized in a single row of cells three great problems of 

 life : its period of duration, struggle for existence, and fatigue. 



No series of changes anywhere in the body have a more 

 direct bearing upon changes during the life history of a nerve 

 cell than this series in a cell of the epidermis. The cells begin 

 life with a large nucleus and little protoplasm exactly as a nerve 

 cell does. Protoplasm grows much, nucleus grows somewhat, 

 like a developing ganglion cell. Farther on, the nucleus begins 

 to shrink, looses its reticular structure, and disappears when the 

 change of the cell from protoplasm to horn has been completed. 

 A similar set of changes have been described for the atrophy of 

 nerve cells, the end product of course being different in the two 

 cases. And whether the life history of nerve and epithelial cells 

 is comparable to the end remains to be seen when the changes 

 due to aging have been fully worked out for the nerve cell. 

 We clearly have in the epidermis functional activity involving 

 the destruction of cells. This fact finds a natural explanation 

 in the superficial position of the cells. Why this should not be 

 the case with nerve cells will be discussed later. 



A single case of marked changes in epidermal cells due to 

 artificial stimulation is given us by Kodis (28) for the tadpole. 

 Kodis finds that one hour's electrical stimulation of the skin 

 occasions a shrinkage in the epidermal nuclei of nearly sixty per 

 cent (figuring volume of nuclei from measurements taken from 

 Kodis' drawings) (compare Taf. Ill, Fig. 34 with Taf, I, Fig. i). 

 The nucleus at the same time becomes granular and dark. The 

 nucleolus also shrinks and ceases to stain bright red with safra- 

 nin as it does in the resting cells. With the exception of this 

 last, which is not so clearly demonstrated in my specimens, the 

 changes are quite similar to those taking place upon stimulating 

 the nerve of a spinal ganglion. 



We shall enter now the vast field of nerve literature with an 

 eye single to the subject in hand ; viz. microscopical changes 

 connected with functional activity. Only so much of morpho- 

 logical interest will be cited as is necessary to supply a physical 

 basis for physiological action. In brief, the conception of the 

 minute structure of nerve elements which has satisfied every 



