CHAP, v.] NUTRITION. 461 



The Sources and Distribution of Heat. We have already 

 seen that the conception of the non-nitrogenous portions of food 

 being solely calorifacient or respiratory proves to be unfounded 

 when we attempt to trace the history of the food on its way 

 through the body. The same view is still more strikingly shewn 

 to be inadequate when we study the manner in which the heat 

 of the body is produced. We may indeed at once affirm that 

 the heat of the body is generated by the oxidation, not of any 

 particular substances, but of the tissues at large. Wherever 

 metabolism of protoplasm is going on, heat is being set free. In 

 growth and in repair, in the deposition of new material, in the 

 transformation of lifeless pabulum into living tissue, in the con- 

 structive metabolism of the body, heat may be undoubtedly to 

 a certain extent absorbed and rendered latent: the energy of the 

 construction may be, in part at least, supplied 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 (p. 70) 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 what is somewhat below 

 the mean result 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 at those times only when they 

 are contracting, must form a very large part of the total heat 

 given out by the body. 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 thermo- 

 genic tissues. 



