364 , HUMAN PHYSIOLOGY 



undergoes only small variations, because the balance between 

 production of heat and elimination of heat is so well main- 

 tained. But under abnormal conditions the balance is 

 frequently upset. Thus severe muscular work causes a 

 temporary rise of temperature, because heat elimination does 

 not quite keep pace with heat production. Exposure to very 

 high temperatures may cause a slight rise of temperature, 

 while exposure to excessive cold may cause a slight fall; 

 but unless in the case of those unable to use their muscles 

 e.g. in those suffering from alcoholic poisoning the change 

 is small. 



Although the normal variation of temperature is so small, 

 life may be sustained when the temperature falls for a time 

 to about 25 C., or rises to nearly 43 C. Cases are recorded in 

 which it has even risen to 46'6 C., without death supervening. 



While the higher "warm-blooded animals," mammals and 

 birds, maintain a constant temperature, the lower vertebrates, 

 " cold-blooded animals," reptiles, amphibia and fishes, do not 

 do so, and their temperature varies with that of the surround- 

 ing medium. 



But even in mammals the mechanism for the regulation of 

 temperature is not absolutely perfect, and in every species 

 of animal there is a limit to the power of adjustment. 



Mammals which hibernate become for the time " cold- 

 blooded animals," and lose their power of regulating their 

 temperature. 



The regulation of temperature may be effected either 

 by modifying heat production, or by altering the rate of 

 elimination. 



Heat production is voluntarily modified when muscular 

 exercise is taken during exposure to cold, and involuntarily 

 when muscles are set in action by a shivering fit. There is, 

 however, no evidence of the existence of a special nervous 

 mechanism presiding over heat production in muscle. 



But it is not so much by changes in the rate of heat pro- 

 duction, as by alteration in heat elimination through the 

 skin, that the temperature is kept uniform. The nerves to 

 the cutaneous vessels, and to the sweat glands, are the great 

 controllers of temperature. It is through failure of this 

 mechanism under the action of the toxins of micro-organisms 



