CHAP, v.] NUTRITION. 639 



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 ( 62) of the thermal changes in muscle we 

 have seen that in the total energy expended in a muscular con- 

 traction, the ratio of that which appears as heat to that which 

 appears as external work is variable. If we assume that the 

 energy appearing as work done in a muscular contraction is 

 on the average 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 eco- 

 nomically 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, 

 120, 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 secret- 

 ing 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 will naturally give rise to heat. 

 In the case of the salivary gland a rise of temperature has been 

 actually observed ; but objections have been brought against 

 the observation. 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 con- 

 clude 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 



