THE CELLULAR CHANGES OF AGE 67 



so called. 1 This is demonstrated by the slide now be- 

 fore us (Fig. 22), which shows corresponding motor 

 nerve cells from the spinal cords of twelve different 

 mammals arranged in the order of their size the ele- 

 phant, the cow, the horse, man, the pig, the dog, the 

 baboon, the cat, the rabbit, the rat, the mouse, and a 

 small bat. You recognise immediately that there is 

 a proportion between the size of these cells and the 

 size of the respective species of animals. To a minor 

 degree, but much less markedly, there is a difference 

 in the calibre and length of the striated or voluntary 

 muscle fibres. But with these exceptions our state- 

 ment is very nearly exactly true, that the difference 

 in size of animals does not involve a difference in the 

 size of their cells. For the purpose of the study of 

 development, which we are to make in these lectures, 

 this uniformity in the size of cells is a great advan- 

 tage, and enables us to speak in general terms in 

 regard to the growth of cells, and renders it superflu- 

 ous to stop and discuss for each part of the body 

 the size of the cells which compose it, or to seek to 

 establish different principles for different animals 

 because their cells are not alike in size. 



Now we pass to a totally different aspect of cell 

 development, that which is concerned with the degen- 

 eration of cells. For we find that, after the differ- 

 entiation has been accomplished, there is a tendency 



1 Irving Hardesty, " Observations on the Medulla Spinalis of the Elephant, 

 with Some Comparative Studies of the Intumescentia Cervicalis and the 

 Neurones of the Columna Anterior," Journ. Comp. Neural., xii, 125-182, 

 pis. ix-xiii. 



