THE ENERGY OF THE BODY. 497 



dered latent ; the energy of the construction may be, in part at least, sup- 

 plied by the heat present. But all this, and more than this, viz., the heat 

 present in a potential form in the substances themselves so built up into the 

 tissue, is lost to the tissue during its destructive metabolism ; so that the 

 whole metabolism, the whole cycle of changes from the lifeless pabulum 

 through the living tissue back to the lifeless products of vital action, is 

 eminently a source of heat. 



Of all the tissues of the body the muscles, not only from their bulk, 

 forming as they do so large a portion of the whole frame, but also from the 

 characters of their metabolism, must be regarded as the chief sources of 

 heat. 



In treating ( 65) of the thermal changes in muscle we have seen that 

 in the total energy expended in a muscular contraction, the ratio of that 

 which appears as heat to that which appears as external work is variable. 

 If we take a proportion which is 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 con- 

 tracting, 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. More- 

 over, to the skeletal muscle we must add the heart which, never resting, 

 does in the twenty-four hours, as we have seen ( 127), no inconsiderable 

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

 But the skeletal muscles, though frequently, are not continually contract- 

 ing ; 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 metab- 

 olism 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 thertnogenic 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 40.73 C. has been observed in the hepatic vein, 



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