CHAP, v.] NUTRITION. 845 



metabolism. But this, like the capital of heat present in a poten- 

 tial form in the substances themselves so built up into the tissue, 

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

 metabolism as a whole, the cycle of changes as a whole 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 a muscle in doing work also gives rise to heat, and 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 accept the estimate there given which 

 makes the muscle work in a most uneconomical way, and we 

 accordingly assume that the energy which appears as work done 

 in a muscular contraction is only one twenty-fifth of the total 

 energy expended, we arrive at the startling result that the muscles 

 alone during their contractions provide far more heat than the 

 whole amount given out by the body. For the muscles by 

 their contractions do all the external work of the body; but the 

 energy represented by the total heat lost to the body in a given 

 time, say twenty-four hours, is as we have seen ( 528) not twenty- 

 four twenty-fifths but only some four-fifths or so of the total 

 energy expended. That is to say, the muscles not only provide all 

 the heat of the body, but a very great deal of the heat which they 

 produce must be used up, absorbed in synthetical processes, 

 within the body, and hence not lost to the body immediately as 

 heat. This result may of course be taken as a strong argument 

 shewing that the above estimate is a wrong one ; for, though we 

 have no exact knowledge of the quantity of heat absorbed in the 

 building up of living tissue, muscle for instance, from lifeless food, 

 we cannot suppose it to be so great as the above demands. But we 

 may also draw from the result the conclusion that even if we take 

 a far more favourable view of the economical working of the 

 muscles, these in their contractions must serve as the chief sources 

 of heat. Indeed even if we suppose that the muscular machine 

 works so economically as to utilize as much as half the energy 

 expended, 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, 

 however economically it may work, must give rise to no in- 

 considerable 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 



