TEMPERATURE REGULATION IN HOMOIOTHERMS 57 



evidently not true in the case of the homoiotherms. 

 Many experiments show that a fall in the temperature 

 of the environment of a warm-blooded animal induces 

 a higher rate of metabolism and with a rise in tempera- 

 ture the reverse seems to be true until the environmental 

 temperature reaches about 35 deg. C, but above this 

 temperature there is again an increase. 



Thermogenesis operates in the body of a homoio- 

 therm to compensate for heat lost by radiation and con- 

 duction. Since the temperature of a homoiotherm is 

 usually above that of its environment, it will constantly 

 lose heat. The amount of heat lost will be determined 

 by the factors influencing radiation and conduction. 

 These have been described in Chapter IV. Thermoge- 

 nesis is thus in a sense a curative mechanism, since it 

 furnishes the means whereby heat is produced in the 

 body to compensate for that lost. 



The energy which is required to maintain life proc- 

 esses at any given temperature and the additional heat 

 which is necessary with a fall in the temperature of the 

 surrounding medium are directly derived from metabolic 

 processes. Heat is liberated through oxidation, either 

 by voluntary muscular exercise, reflex muscular con- 

 tractions such as shivering, or by the transformation of 

 food. Oxidations take place in the tissues and the seat 

 of heat production is therefore in them. The greater 

 part of the body heat of a mammal is produced in the 

 muscles. Seventy to 80 per cent of the energy liberated 

 during a muscle contraction is in the form of heat. About 

 one-half the active tissues of ^n average vertebrate are 

 muscular and, as the muscles are by far the most active 

 of the tissues, a great amount of heat must be furnished 

 by them. Some of the glands, especially the liver, are 

 also important sources of heat. 



