1335 



greater volume than tlie cell-body in which it arises^); this becomes 

 374 times, when the medullary sheath, which if not genetically, 

 at any rate physiologically belongs to the nerve fiber, is included '). 



Now the question rises whether there also exists a regular relation 

 between the volume of the cell-body and the entire neurone. 



The length of the nerve fiber which connects a cell-body, e.g. 

 in the spinal cord, with the periphery of the body, must naturally 

 increase with the size of the animal species, indeed about propor- 

 tional to the length of the body. That also the caliber of the 

 nerve fiber is to some extent dependent on the size of the body, 

 Cfjeteris paribus, was repeatedly demonstrated, perhaps most convin- 

 cingly by Hardesty *) by means of a number of measurements in 

 the same sections of the spinal cord in which the cell-bodies 

 were measured, for the elephant, for tnan, and for the mouse. The 

 area of the section of the nerve fiber appears aeteris paribus to 

 grow about equally with that of the cell-body. 



The subjoined series rests on calculatiojis of the exponent of relation 

 for the volume of the cell-body in proportion to that of the 

 entire neurone, the medullary sheath included. ^) The latter on 

 the ground of what Donaldson and Hoke have shown and in view 

 of the comparison with the quantity of the brain. 



Everywhere the largest neurone, with the longest neurite, of 

 the hand is concerned. 



It then appears that : 



') H H Donaldson, The Growth of the Brain. London 1895, p. 154. 



') According to ttie result of Donaldson and Hoke's research, I. c 



3) L. c. p. 171-172, Compare also: G. Levi, 1. c. p. 338, who measured the 

 thickest nerve fibers of the sciatic. For animals that do not differ much in 

 size, the change is by no means regular (for reasons that will be discussed 

 later on). 



*) The cell-body was considered as a sphere with the indicated diameter; in 

 the calculation of the volume of the entire neurone collateral branches and the 

 greater portion of the dendritic processes were neglected 



6) The figure placed after this rat rests on a calculation from data of 



