CHAP, v.] NUTRITION. 809 



that the blood of the hepatic vein is warmer than that of either 

 the portal vein or the aorta, shews that the increased temperature 

 is not due simply to the liver being far removed from the surface of 

 the body. 



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

 temperature 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 or other metabolic changes taking place 

 in it are comparatively slight. The heat evolved by the in- 

 different tissues such as bone, cartilage and connective 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 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 in all probability 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 

 1" ^ii also called poikilothermic, that is, of varied temperature. 

 Exceptions among them are not uncommon. Some fish, such as 



