8 Journal of Comparative Neurology and Psychology. 



as the standard, it is less by 0,6%. In this particular group, 

 therefore, the actual area of the sheath is 0,6% greater than it 

 would be if the assumed one to one relation of the axis and 

 sheath were maintained. 



In working out the results, it is this ideal of one-half the 

 area of the total fiber which is always taken as the standard, 

 and the observed area of the axis is compared with it. If the 

 area of the axis is less than one-half the area of the total fiber, 

 then it follows that the sheath must have been mor^ than one- 

 half and the percentage value of the difference is written in plus, 

 to show that the sheath is too large by this amount, or, 

 under the reverse conditions, as minus, to show that it is too 

 small. This difference is designated the average percentage 

 deviation. 



In the case of each of the 1540 fibers here presented (Ta- 

 ble VI) calculations similar to those just given, have been made. 

 It is thus possible to say in each instance by what percentage 

 the area of the sheath departs from that of the ideal, although 

 in the table only the percentages for the extreme cases and for 

 the average deviation are given. 



Description of the Material Employed. — The following list 

 of the specimens gives the common name ; scientific name (en- 

 tered only once where several specimens of the same species 

 were examined); length of body; weight; sex; age; season when 

 killed; nerves taken; locality; and by whom prepared. Where 

 no statement is made; the sex is male; the season winter; the 

 nerves are taken from the brachial plexus, and the material 

 killed and prepared by Mr. Hoke in this laboratory. The omis- 

 sion of any of the other data means that they are not available. 



