524 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



an impulse through the central nervous system is to initially facilitate 

 and ultimately retard the passage of subsequent impulses along the 

 same path. The nature of the initial facilitation has been variously 

 characterized. Thus Maudsley described it as the formation of a 

 trace or thread of a deposit which is followed by the succeeding impulse, 

 while Exner likened it to the "excavation of a channel/' a hypothesis 

 which is generally referred to as the Canalization Hypothesis. 



In preceding chapters we have had frequent occasion to dwell upon 

 a variety of chemical processes and not a few life-phenomena which 

 display initial facilitation followed by retardation. These are the 

 various processes or phenomena which are governed as to their speed 

 by underlying Autocatalyzed Reactions. It is evident that if the passage 

 of an impulse through the central nervous system were attributable to 

 the occurrence of an autocatalyzed chemical reaction, the deposition 

 of the products of this reaction along the path of the impulse would 

 facilitate the passage of a subsequent impulse, while their accumula- 

 tion in undue amount would constitute an impediment to the further 

 occurrence of the reaction and therefore to the passage of subsequent 

 impulses. The same mechanism thus accounts for both the facilitation 

 and the fatigue which accompany the performance of functions involv- 

 ing the central nervous system. 



Regarding the nature of the autocatalyst in this reaction we are of 

 course completely in the dark in so far as any direct results of chemical 

 analysis are concerned. We may, however, draw certain more or less 

 probable inferences from our knowledge of the behavior of a par- 

 ticular part of the central nervous system, namely, the Respiratory 

 Center. In this region we have a rhythmic passage of impulses of which 

 the frequency is determined by the alternate facilitation and retarda- 

 tion of conduction which is brought about, as we have seen in a pre- 

 ceding chapter, by the presence of greater or lesser amounts of Lactic 

 Acid, Carbon Dioxide, or other fatty or hydroxy fatty acids in the cir- 

 culating fluids. Evidently, therefore, acids, or at least this particular 

 class of acids, facilitate the passage of impulses through this if not 

 through other regions of the nervous system. Now hyperactivity of 

 the central nervous system results in the accumulation of acid sub- 

 stances in the brain, and we may with some probability infer that the 

 normal activities of the central nervous system are accompanied to a 

 lesser degree by the production of similar substances. 



THE FATIGUE-PRODUCTS OF NERVE-CENTERS. 



It has been pointed out by Mosso that the fatigue-products of 

 Nerve-centers and those of Muscle are probably very similar in nature 

 since mental fatigue is accompanied by signs of muscular fatigue and 

 vice versa. Among the products of muscular activity two acids figure 

 very largely, namely Lactic Acid and Carbonic Acid, and, if the products 

 of muscular and of nerve-cell activity are similar, we should expect to 



