808 SOURCES OF HEAT. [BOOK 11. 



somewhat higher than the mean of the range there given (one 

 fifth to one twenty-fifth), and assume that the energy involved in 

 the work done in a muscular contraction is about one-tenth of 

 the total energy expended, the rest going out as heat, then, upon 

 the calculation that the total external work of the body is about 

 one-fifth of the total energy set free in the body, it is clear that the 

 heat given out by the muscles, even if we consider only the heat 

 given out when they are contracting, must form a very large part 

 of the total heat given out by the body. And even if, as recent 

 researches indicate, the muscular machine works more economically 

 than we have hitherto supposed, the amount of heat given out by 

 the skeletal muscles must still remain very large. Moreover to the 

 skeletal muscle we must add the heart which, never resting, does 

 in the twenty-four hours as we have seen, 138, no inconsiderable 

 amount of work, and must give rise to no inconsiderable amount 

 of heat. But the skeletal muscles, though frequently, are not 

 continually contracting; they have periods, at times long periods, 

 of rest ; and during these periods of rest, metabolism, of a subdued 

 kind it is true, but still a metabolism 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 knowledge 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, and 

 in all probability the investigation of other secreting glands would 

 lead to similar results. 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 40'73C. has been 

 observed in the hepatic vein, while that of the vena cava inferior 

 was 38-35 to 39'58, and that of the right heart 377. The fact 



