CHAP, v.] NUTRITION. 641 



the frog, for instance, is rarely more than -04 to *05 above 

 that of the atmosphere, though in the breeding season the 

 difference may amount to 1. Such animals, and they comprise 

 all classes except birds and mammals, are spoken of as cold- 

 blooded ; they have been also called poikilothermic, that is, of 

 varied temperature. Exceptions among them are not uncom- 

 mon. Some fish, such as the tunny, are warmer than the water 

 in which they live, and in a species of Python (P. bivittatus) a 

 difference of as much as 12 has been observed. In a beehive 

 the temperature may rise at times to as much as 40. In the 

 so-called warm-blooded animals, birds and mammals, the loss 

 and production of heat are so balanced that the temperature of 

 the body remains constant at, in round numbers, 35 or 40, 

 whatever be the temperature of the air ; hence these have 

 been called homoiothermic, of constant temperature. The 

 temperature of man is about 37 ; in some birds it is as high 

 as 44 (Hirundo) and in the wolf it is said to be as low as 

 35-24. 



This temperature is with slight variations maintained 

 throughout life. After death the generation of heat rapidly 

 diminishes, and the body speedily becomes cold ; but for some 

 short time immediately following upon systemic death, a rise 

 of temperature may be observed, due to the fact that, while the 

 metabolism of the tissues is still going on, the loss of heat is 

 somewhat checked by the cessation of the circulation. The 

 onset of pronounced rigor mortis causes a marked accession of 

 heat, and when occurring after certain diseases may give rise 

 to a very considerable elevation of temperature. 



This mean bodily temperature of warm-blooded animals is, 

 during health, maintained, with small variations of which we 

 shall presently speak, within a very narrow margin, a rise or 

 indeed a fall of much more than a degree above or below the 

 limit given above being indicative of some failure in the organ- 

 ism, or of some unusual influence being at work. It is evident, 

 therefore, that the mechanisms which coordinate the loss with 

 the production of heat must be exceedingly sensitive. It is 

 obvious, moreover, that these mechanisms may act when the 

 bodily temperature is tending to rise, by either checking the 

 production or by augmenting the loss of heat ; conversely when 

 the bodily temperature is tending to fall, they may act by either 

 increasing the production or by diminishing the loss of heat. 

 As the regulation of temperature by variations in the loss of 

 heat is better known than regulation by variations in produc- 

 tion, it will be best to consider this first. 



426. Regulation by variations in loss. Heat is lost to the 

 body by the warming of the faeces and of the urine, by the 

 warming of the expired air, by the evaporation of the water 

 of respiration, by conduction and radiation from the skin, and 



41 



