CHAP, v.] NUTRITION. 847 



by which its latent energy is set free. Some amount of heat is 

 also produced by the changes which the food undergoes in the 

 alimentary canal before it really enters the body. 



Hence, taking a survey of the whole body, we may conclude 

 that since metabolism is going on to a greater or less extent 

 everywhere, heat is everywhere being generated ; but that, looked 

 at from a quantitative point of view, the muscles and the glandu- 

 lar organs must be regarded as the main sources of the heat of the 

 body, the muscles being the more important of the two. 



531. But heat, while being thus continually produced, is as 

 continually being lost, by the skin, the lungs, the urine and the 

 faeces. The blood passing from one part of the body to the other, 

 and carrying warmth from the tissues where heat is being rapidly 

 generated, to the tissues or organs where heat is being lost by 

 radiation, conduction or evaporation, tends to equalize the tempera- 

 ture of the various parts, and thus maintains a " constant bodily 

 temperature." 



When the production of heat is not great as compared with the 

 loss there is no great accumulation of heat within the body, the 

 temperature of which consequently is but slightly raised above 

 that of surrounding objects. Thus the temperature of 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 uncommon. 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 as much as to 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 through- 

 out 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. 



