136 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



direct proof that this property is possessed universally by the nerve 

 cells nor any indication of the probable 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 

 that in functional activity acid is also produced. Mosso states 

 that in the brain increased mental activity is accompanied by a 

 rise in the temperature of the brain, f His experiments were made 

 upon individuals with an opening in the skull through which a 

 delicate thermometer could be inserted so as to lie in contact with 

 brain. So also the facts briefly mentioned in regard to the Nissl 

 granules give some corroborative evidence that the activity of 

 the nervous system is accompanied by and probably caused by 

 a chemical change within the cells, since the excessive activity of 

 the nerve cells seems to be accompanied by some change in these 

 granules, and in abnormal conditions associated with loss of func- 



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

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

 t Mosso, "Die Temperatur des Gehirns," 1894. 



