No. 2.] CHANGES IN NERVE CELLS. 143 



be somewhat irritable in sleep ^ (43, p. 53 ; 58, p. 345; 39, p. 



693). 



A second dog, stimulated similarly for one hour and twenty- 

 five minutes (10.05 to 11.30 a.m.), dying suddenly from the oper- 

 ation on the brain at the end of this time, showed most clearly 

 the characteristic effects of fatigue. Hence, we are not com- 

 pelled to make an exception in the case of dogs. 



It is plain from the above considerations that a study of the 

 normal rhythm of sleep and activity should be made for the 

 animal employed in connection with further work of this kind. 

 To this end I have kept under constant observation for a week 

 a half-grown kitten similar to the ones used in my experiments. 

 The sleep of such a kitten depends largely upon the amount of 

 food given to it. If fed to repletion, it would sleep as much 

 as eighteen hours a day, and, even when sparingly fed, slept 

 twelve and one-half or thirteen hours. It seemed to be able to 

 sleep equally well day or night. In short, the curve of nervous 

 activity of a cat is most irregular. 



It will be noted that if the cat possesses no marked daily 

 rhythm of rest and activity, our provisional curves are more 

 likely to be correct. 



VII. Effects of Normal Daily Fatigue. 



A crucial test as to the value of foregoing experiments for 

 normal physiology is readily seen to lie in the question. Do 

 changes in ganglion cells, like those observed during artificial 

 stimulation, actually occur in the normal activity of an animal ? 

 If they do not, the experiments do not concern normal physi- 

 ology of the nervous system. In spite of all proof to the con- 

 trary, they must be considered pathological. If they do occur, 

 with the evidence already adduced, it will be but fair to consider 

 them a part of the normal physiological activity of the nervous 

 system. 



1 This experiment may have been further complicated by the fact that the pup, 

 being of large breed and growing rapidly, lymph in great amount exuded from the 

 wound and formed a pool in the axilla around the nerves and electrode. I did 

 not notice this until quite late, when I thought that the contractions were becoming 

 weak from fatigue. On wiping up the lymph more carefully, they became as strong 

 as at first. In short, stimulation may not have been as " intense " as I had designed 

 to have it. 



