498 NUTRITION. 



while that of the vena cava inferior was 38.35 to 39.58, and that of the 

 right heart 37.7. The fact that the blood of the hepatic vein is warmer 

 than that of either the portal vein or the aorta, shows that the increased 

 temperature is not due simply to the liver being far removed from the sur- 

 face of the body. 



The brain, too, may be regarded as a source of heat, since its tempera- 

 ture is higher than that of the arterial blood with which it is supplied ; 

 though from the smaller quantity of blood passing through its vessels, as 

 well as from the changes in it being less massive, it cannot, in this respect, 

 compare with either the liver or the muscles as a source of heat to the 

 body. 



The blood itself cannot be regarded as a source of any considerable 

 amount of heat, since, as we have so frequently urged, the oxidations 01 

 other metabolic changes taking place in it are comparatively slight. The 

 heat evolved by the indifferent tissues, such as bone, cartilage, and connec- 

 tive tissue, may be passed over as insignificant ; and we cannot even regard 

 the adipose tissue as a seat of the production of heat, since the fat of the 

 fat-cells is in all probability not oxidized in situ, but simply carried away 

 from its place of storage to the tissue which stands in need of it, and it is in 

 the tissue that it undergoes the metabolism by which its latent energy is set 

 free. Some amount of heat is also produced by the changes \vhich 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 every- 

 where being generated ; but that, looked at from a quantitative point of view, 

 the muscles and the glandular organs must be regarded as the main sources 

 of the heat of the body, the muscles being, in all probability, the more im- 

 portant of the two. 



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

 being lost, by the skin, the lungs, the urine, and the feces. The blood pass- 

 ing 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 temperature 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 0.04 to 0.05 

 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 tem- 

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

 whatever be the temperature of the air ; hence these have been called homoio- 

 thermic, 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 



