CHAP, i.] OF ORGANIC HEAT. 447 



it depends then much more upon tho biological conditions of the 

 individual than upon the ambient medium. If it resist refrigerant 

 influences, it can also, in a certain degree, resist those of warmth, 

 and maintain itself for a certain time below the exterior tem- 

 perature when the latter is excessive. The principal means of 

 resistance to heat is evaporation, which proceeds on the cutaneous 

 and respiratory surfaces. 



The maintenance of the median temperature of the blood in birds 

 and mammifers is a primordial condition. Thermic deviations, even 

 when very slight, immediately put life in peril. The tempera- 

 ture of the blood cannot with impunity go beyond 43 degrees 

 in the mammifers, and 50 degrees in birds. At 42 degrees the 

 blood may already coagulate in the vessels ; at 49 or 50 degrees 

 the muscular substance coagulates ; the muscles are then rigid 

 and acid. 1 



As the composition of the albuminoidal substances is much 

 less alterable by cold than by heat, the lowering of the organic 

 temperature is less dangerous than its elevation; at least 

 there is, herein a larger margin. Small mammifers (rabbits, 

 guinea-pigs) make cold rapidly in ice, die when the temperature 

 of their rectum descends to 18 or 20 degrees. But, by proceeding 

 slowly, gradually, it is possible to lower their temperature much 

 more without killing them, since, in marmots, the temperature of 

 the rectum during hibernation is only 4 or 5 degrees. 



We know that all animals absorb oxygen in quantities which 

 vary according to determinate laws, with the class, the zoo- 

 logical species, the physiological conditions. We know, on the 

 other hand, that this absorbed oxygen is the principal agent in 

 the chemical mutations and transformations which are at once 

 the effect and the cause of nutrition. Now all combustion is 

 developed from heat. We can, then, taking the foregoing general 

 facts as our sole basis, assign as the cause of the production and 

 maintenance of organic temperatures, the oxydation, the slow. 



1 Cl. Bernard, Rapport sur les Progres de la Physiologic, p. 45, and Her- 

 mann, Elements de Physiologic, Paris, 1869. 



