130 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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. Evidence of a 

 probable chemical change in the nerve cells during activity is found 

 also in the readiness with which the gray matter of the nervous 

 system takes on an acid reaction.* In the fresh resting state it 

 is probably 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 hi 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- 

 tional activity the granules undergo chromatolysis, that is, they 

 are disintegrated and dissolved. Obvious histological changes which 

 imply, of course, a change in chemical structure, have been observed 

 by a number of investigators.:]: All seem to agree that activity of 

 the tissue, whether normal or induced by artificial stimulation, 

 may cause visible changes in the appearance of the cell and its 

 nucleus. Activity within normal limits may cause an increase in 

 the size of the cell together with a diminution in the stainable 

 (Nissl) substance, and excessive activity a diminution in size of the 

 cell and the nucleus, the formation of vacuoles in the cell body, 

 and a marked effect upon the stainable material. Hodge has 

 shown that in birds, for instance, the spinal ganglion cells of a 

 swallow killed at nightfall after a day of activity exhibit a marked 

 loss of substance as compared with similar cells from an animal 

 killed in the early morning (Fig. 60). 



It must be remembered, however, that our knowledge of the 

 nature of the chemical changes that occur in the cell during activity 

 is very meager. Presumably carbon dioxid and lactic acid are 



* Lan^endorff, "Centralbl. f. d. med. Wiss.," 1886. See also Halliburton, 

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



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



} See especially Hodge, "Journal of Morphology," 7, 95, 1892, and 

 9, 1, 1894. 



