240 IRRITABILITY 



A further very interesting example of depression produced 

 by oxygen deficiency is furnished by heat depression. It has 

 long been known that with increasing temperature the vital mani- 

 festations of all poikilothermic organisms at first undergo a 

 heightening of their intensity. If, however, after a maximum 

 is reached, the temperature is still further increased a sudden 

 depression sets in. The increase in the rapidity of the vital pro- 

 cess as a result of increased temperature is readily understood 

 when based on the well-known law discovered by varit Hoff. 

 Numerous investigations on the rapidity of the course of special 

 vital manifestations, as, for instance the growth of the eggs of 

 the frog and sea urchin, the assimilation of carbon dioxide in 

 green plant cells, the number of vacuole pulsations in the infu- 

 soria cells, the frequency of the heart rate of the frog and of the 

 mammal, etc., have shown that their increase does in fact follow 

 the van't Hoff law, being doubled or tripled in amount with every 

 increase of ten degrees of temperature. The genesis of depres- 

 sion produced by heat, developed in different organisms at various 

 heights of temperature, requires a closer analysis. This depres- 

 sion takes place at temperatures below that in which coagulation 

 of proteins occurs. Therefore, under certain conditions, with 

 which we shall presently become acquainted, it is capable of being 

 recovered from, whereas in higher temperatures, in which albu- 

 men coagulates, vital activity is permanently obliterated. Depres- 

 sion produced by heat is, therefore, in itself not a necrobiotic 

 process, which, as such, must necessarily lead to death. But 

 rather like fatigue it must be looked upon as an asphyxiation 

 process. Its relations to oxygen exchange have been chiefly 

 demonstrated by Wintersteiri 1 by his investigations on the central 

 nervous system of frogs and on medusae. He found that when 

 placed in a heated chamber in a temperature of 32-40 the 

 activity and reflex excitability of the frog are at first augmented. 

 Within the lapse of a short time this increase has become so 

 great that the slightest touch produces tetanic contractions, simi- 



1 H. Winter siein: "Ueber die Wirkung der Warme auf den Biotonus der Nerven- 

 zentren." Zeitschr. f. allgem. Physiol. Bd. I, 1902. The same: "Warmelahirmng und 

 Narkose." Ibid. 



