170 Journal of Comhakativk Nkukologv 



From what we know of tlic sizes of the cells of the 01 her 

 tissue systems of the different mammals, one can scarcely ex- 

 pect a variation in the size of the cells of the nervous system 

 in marked proportion to the variations in body weight. While 

 it may be claimed that there is a slight proportional variation 

 in the size of the tissue cells among the mammals, such a claim 

 would be wholly untenable were the whole of the \ erUbrata 

 included. Both Cavazzani's and Buhlek's measurements for 

 the frog show it to possess a nerve cell body larger in proportion 

 to its body weight than is possessed by any of the mammals. 

 And it is further well known that most of the tissue elements 

 of the still lower and small vertebrates are larger even than the 

 corresponding tissue elements of the higher vertebrates. 



But it must be remembered that with reference to the neu- 

 rone, the size of the cell-body is but a feeble expression of the 

 real size of the neurone. The difference brought out in the 

 above tables can be little more than indicative. The neurone 

 is a peculiarly differentiated tissue element in which the far 

 greater part of the cell substance is sent out in more or less far 

 reaching processes. Donaldson ('98) has made computations 

 showing the relation between the volume of the cell-body of 

 certain of the neurones of the central nervous system of man 

 and the volume of its longest process, the axone, and has found 

 that the axone may have a volume 187 times that of the cell- 

 body from which it is an outgrowth. This computation of 

 course does not include the medullary sheath of the axone which 

 must be considered as an acquired structure. One must re- 

 member therefore that the elephant, while having in its intu- 

 mescentia cervicalis a cell body appreciably larger than any of 

 the other mammals investigated, must have an entire neurone 

 whose total volume exceeds that of the other mammals to a far 

 greater extent than is indicated by the mere diameter of the 

 cell body. 



The majority of the large cell bodies (those measured) of 

 the intumescentia cervicalis are situated in the columna lateralis 

 (lateral horn). These innervate (Kaiser ('91) Collins ('94) 

 Sang ('97) ) the muscles of the fore leg. An axone, then, 



