PROPERTIES OF THE NERVE CELL. 135 



this latter point, it is believed in physiology that the nerve cells do 

 show fatigue. The nerve centers fatigue as the result of continuous 

 activity, as is evident from our personal experience in prolonged in- 

 tellectual or emotional activity and as is implied in the necessity of 

 sleep for recuperation and by the rapidity with which functional 

 activity is lost on withdrawal of the blood supply. Objectively, 

 also, it has been shown in the ergographic experiments (see p. 50) 

 that the well-known fatigue of the neuromuscular apparatus 

 affects the nerve centers as well as the muscle. Assuming that 

 the nerve cells are the effective agent in the nerve centers, such facts 

 indicate that they are susceptible to fatigue under what may be des- 

 ignated as the normal conditions of activity. But we have no very 

 direct proof that this property is possessed universally by the 

 nerve cells nor any indication of the differences in this regard 

 shown by nerve cells in different parts of the central nervous system. 

 It seems probable that under normal conditions — that is, under 

 the influence of what we may call minimal stimuli — some portions 

 of the nerve centers remain in more or less constant activity during 

 the day without showing a marked degree of fatigue, just as our 

 muscles remain in a more or less continuous state of tonic con- 

 traction throughout the waking period at least. Doubtless when 

 the stimulation is stronger the fatigue is more marked, because the 

 processes of repair in the nerve centers can not then keep pace 

 with the processes of consumption of material. In general, it 

 may be held that every tissue exhibits a certain balance between 

 the processes of consumption of material associated with activity 

 and the processes of repair. If a proper interval of rest is allowed, 

 the tissue will function without exhibiting fatigue, as is the case 

 with the heart and the respiratory center. If, however, the stimu- 

 lation is too strong or is repeated at too brief an interval, then the 

 processes of repair do not keep pace with those of consumption, 

 or the products of functional activity are not completely removed, 

 and in either case we have the phenomenon of fatigue, that is to say, 

 a depression of normal irritability. The point of importance is 

 to determine the differences in this respect between the. different 

 tissues. Our actual knowledge on this point as regards nerve 

 cells is quite incomplete. Evidence of a probable chemical 

 change in the nerve cells during activity is found in the readi- 

 ness with which the gray matter of the nervous system takes 

 on an acid reaction.* In the fresh resting state it is prob- 

 ably alkaline or neutral, but after death it quickly shows an 

 acid reaction, due, it is said, to the production of lactic acid. Its 

 resemblance to the muscle in this respect leads to the inference 



*Langendorff, "Centralbl. f. d. med. Wiss.," 1886. See also Halliburton, 

 "The Croonian Lectures on The Chemical Side of Nervous Activity," 1901. 



