172 Journal of Comparative Neurology. 



of the fingers. Then the axone extending this distance has a 

 volume of 44, 312,000 cu. micra or 434 times the volume of 

 the cell body giving origin to it. Adding 102,109 cu. micra 

 (the volume of the cell body with a diameter of 58 micra) to 

 the volume of the axone, gives the entire neurone a volume of 

 44,414, 109 cu. micra. By the necessary divisions it will be 

 found that the motor neurone of the cervical region of the ele- 

 phant may be 5.2 times that of man, while the diameter of the 

 corresponding cell-bodies is only 1.3 times that of man. The vol- 

 ume of the cell body of the elephant, the area of the corres- 

 ponding section of the medulla spinalis of the elephant, and the 

 area of the section of the substantia grisea are, all three, about 

 3 times as great as in man though the body weight of the ele- 

 phant is 50 times as great as man's. These coincident ratios 

 between the elephant and man seem, however, to be exceptional 

 for the series. 



For the mouse the corresponding cell-body having a mean 

 diameter of 27.4 micra (Table III) would have a volume of 

 7974 cu. micra. Measurements of the corresponding axones of 

 the radix anterior give them an average diameter of 4.2 micra, 

 and thus an area in section of 13.9 sq. micra. Measurements 

 of eight adult mice give 35 mm. as the conservative average 

 distance between the medulla spinalis and the center of the fore 

 foot. An axone extending this distance would therefore have 

 a volume of 486,500 cu. micra, or 61 times that of the cell 

 body from which it arises. The volume of the axone added to 

 the volume of the cell-body gives 494,474 cu. micra as the vol- 

 ume of the entire neurone. 



Comparing these quantities for the mouse with the corres- 

 ponding quantities for the elephant, it is found that the entire 

 neurone arising in the intumescentia cervicalis of the elephant 

 may be 469 times as voluminous as that of the mouse, while 

 the volume of corresponding cell-body is 39 times as great, and 

 the mean diameter of the cell-body only about 3 times as great 

 as that of the mouse. It has already been shown that the area 

 of the section of the medulla spinalis of the elephant is 154 

 times the area of the corresponding section of the mouse, while 



