242 IRRITABILITY 



of fatigue. In fatigue, a relative want of oxygen is produced by 

 the increased consumption following functional activity, in heat 

 depression by the increase of the entire metabolism producing a 

 corresponding increase of oxygen requirement. In both instances 

 we have an excitation produced by external stimuli which result 

 in an increase in the amount of oxygen required, and in both 

 instances the oxygen at disposal is not sufficient to permanently 

 meet the augmented demand. In both types, therefore, decom- 

 position must become more and more anoxydative and the well- 

 known series of processes is developed, which find their expres- 

 sion in depression. 



In another direction likewise heat depression is of special 

 interest, that is, in regard to the theory of nature of the pro- 

 cesses in the living substance. According to the van't Hoff law 

 we may assume that every individual constituent metabolic pro- 

 cess, if we imagine it as isolated and taking place in a test tube, 

 undergoes in more or less the same degree as all others an in- 

 creased rapidity of reaction as a result of increased temperature. 

 At the same time, in living substance we find on the contrary 

 that the van't Hoff law is only within certain narrow limits more 

 or less applicable to the sum total of all metabolic processes. 

 Beyond certain degrees of temperature no further increase of 

 the vital process takes place, instead a retardation occurs. The 

 analysis of depression produced by heat shows us in the clearest 

 and simplest manner the reason for this apparent deviation from 

 the general law of van't Hoff. This reasoning is based on the fact 

 that the rapidity of reaction of a chemical process is not merely 

 dependent upon the temperature, but likewise upon the mass 

 relations of the reacting substances. In spite of the effect of the 

 temperature in increasing the rapidity of reactions, the process 

 undergoes retardation which extends to a complete cessation if 

 the supply of material necessary to its existence does not keep 

 pace with the increase produced by temperature. In the present 

 instance the amount of reserve supplies for the building up of 

 the disintegrating molecules exists in abundance, and it is merely 

 the available oxygen which is in relatively a very small quantity. 

 As soon, however, as metabolism in its entirety, or even merely in 



