ARRANGEMENT OF STRUCTURAL ELEMENTS. 215 



of the animals. We know, further, that the homologous 

 nerve cells in the spinal cord of the horse and the ox have 

 cell-bodies only slightly larger than those found in man. 

 It is necessary, however, to be on guard against the 

 somewhat misleading character of linear measurements. 

 For example, a cell 53 fju in diameter has, when its 

 volume is calculated as a sphere, only two-thirds the 

 volume of one 59 /z in diameter. The slight difference 

 in diameter is therefore quite significant when trans- 

 lated into volume. 



In this description the expression cell-body has been 

 reiterated for the sake of contrasting the measurements 

 on the body with those which are next to be considered, 

 and which relate to the mass of the entire cell. 

 Supposing the cell-bodies were even of equal volume 

 in the bat and in man, nevertheless owing to the enor- 

 mously larger absolute size of the human nervous 

 system, the neurons must have a much greater length in 

 man than in the bat, making the mass of the entire cell 

 many times greater. The converse is true of the largest 

 mammalia when their nerve elements are compared with 

 those of man, but just how different the relative develop- 

 ment of the nerve cells in the different mammals maybe 

 is still undetermined. Formulating these results, they 

 may be expressed as follows : In the mammalian series 

 cell-bodies do not regularly increase in weight with the 

 increase in the body-weight. On the other hand, the 

 mass of the neuron is more closely correlated with this 

 body- weight, since the larger the animal the greater the 

 length as well as the diameter of the neuron. This 

 latter relation is suggestive, for it follows that in the 

 large animals a very much greater mass of nerve sub- 

 stances, represented by the neuron, is under the control 

 of a single nucleus. At the same time it is not possible 

 to see that there is any other striking difference between 



