846 SOURCES OF HEAT. [BOOK n. 



involving an expenditure of energy, is going on. This quiescent 

 metabolism must also give rise to a certain amount of heat ; and 

 if we add this amount, which in the present state of our know- 

 ledge we cannot exactly gauge, to that given out during the 

 movements of the body, it is very clear, even in the absence of 

 exact data, that the metabolism of the muscles must supply a very 

 large proportion of the total heat of the body. They are par 

 excellence the thermogenic tissues. 



Next to the muscles in importance come the various secreting 

 glands. In these the secreting elements, at the periods of secretion 

 at all events, are in a state of metabolic activity, which activity as 

 elsewhere must give rise to heat. In the case of the salivary gland 

 of the dog the temperature of the saliva secreted during stimulation 

 of the chorda has been found to be as much as 1 or 1*5 higher 

 than that of the blood in the carotid artery at the same time, but 

 the correctness of this observation has been called in question. 

 Of all these various glands, the liver deserves special attention on 

 account of its size and large supply of blood, and because it appears 

 to be continually at work. If there be any truth in the views 

 urged in the preceding chapter touching the large and varied 

 metabolic work of the liver, we must conclude that a very large 

 amount of heat is set free in this organ; and that holds good even 

 if we make a large allowance for the various synthetic anabolic 

 processes which may take place and by which heat would be 

 absorbed and made latent. We find indeed that the blood in the 

 hepatic vein is the warmest in the body. Thus in the dog a 

 temperature of 40'73 C. has been observed in the hepatic vein, 

 at a time when 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, 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 



