12 Journal of Comparative Neurology and Psychology. 



Thus when we take the difference between the sum of all 

 the minus percentai^es and the sum of all the plus percentages 

 by which the sheath differs from the ideal half of the area of the 

 entire fiber, we find that for the 1540 fibers, this amounts on 

 the average to + -45 %• That is, on the average in this series, 

 the area of the medullary sheath exceeds the ideal half by 

 + .45%, or one-thirteenth of ± 6^, the deviai ion which might 

 be expected as an error of observation in the case of a single 

 fiber. 



As tending to cause deviation from the ideal standard we 

 have already noted mechanical injury of the nerve before hard- 

 ening and to this must be added delay in dissection after death. 



In this connection some of the material received through 

 the kindness of Dr. Hardesty calls for special comment — com- 

 ment which is made with the full recognition of the good for- 

 tune which brought the specimens into our hands and is merely 

 intended to indicate how readily the nerves undergo slight alter- 

 ations after death. Specimens 35-38 and 40-44 inclusive were 

 given us by Dr. Hardesty, the animals in all cases coming 

 from the Zoological Garden in San Francisco. 



Dr. Hardesty writes as follows: "In every case the ani- 

 mals from which I sent you bits of nerve were dead when they 

 came into my hands. Most of them came to me within a few 

 hours after death, and none had been dead more than twenty- 

 four hours. In case of the monkeys, most died of tuberculosis, 

 some of the other animals died of pneumonia, one by dysentery^ 

 one by accident and the bear from poison." 



On looking- over our results on this material 



show excessively large sheaths, giving 160 fibers with an aver- 

 age of + 4%, while among the remainder of the mammals from 

 San Francisco, namely the bear, Manila monkey, spider mon- 

 key. Rhesus and baboon the average is only -(-1.14%. We 

 conclude from this that the nerves of the first group had suf- 

 fered a slight post-mortem alteration before they could be fixed. 



