496 NUTRITION. 



In no case has a direct relation between the amount of labor and 

 amount of urea been observed. More than this, the following experience 

 lands us in an absurdity, if we suppose the whole energy of muscular work 

 to arise from proteid metabolism. Two observers performed a certain 

 amount of work (an ascent of a mountain) on a non-nitrogenous diet, and 

 estimated the amount of urea passed during the period. Assuming the 

 urea to represent the oxidation of so much proteid matter, which oxidation 

 represented in turn so much energy set free, they found that, whereas the 

 actual work done amounted to 129.026 and 148.656 kilogramme-kilometres 

 for each observer respectively, the total energy available from proteid 

 metabolism during the period was in the case of the first 68.69, and of the 

 second 68.376 kilogramme-kilometres. That is to say, the energy set free 

 by the proteid metabolism of the muscles engaged in the work was far less 

 than the amount necessary to accomplish the work actually done, to say 

 nothing of its having to provide as well for the movements of respiration 

 and circulation. Their muscular energy, therefore, must have had other 

 sources than proteid metabolism. 



That, on the contrary, the production of carbonic acid is at once and 

 largely increased by muscular exercise is beyond all doubt. One hour's 

 hard labor will increase fivefold the quantity of carbonic acid given off 

 within the hour. And in an experiment directed to this point it was 

 found that a man in twenty-four hours consumed 954 grammes oxygen and 

 produced 1284 grammes carbonic acid when doing work, as against 708 

 grammes oxygen consumed and 911 grammes carbonic acid produced when 

 remaining at rest, the quantity of urea secreted being in the first case 37 

 grammes, in the second 37.2 grammes. 



It is evident that the conclusions arrived at by the statistical method 

 entirely corroborate those gained by an examination of muscle itself, viz., 

 that during muscular contraction the explosive decomposition which takes 

 place bears chiefly, if not exclusively, on the non-nitrogenous constituents 

 of the muscle, and that it is the non-nitrogenous products which alone 

 escape from the muscle and from the body, any nitrogenous products which 

 result being retained within the muscle, or at least within the body. We 

 must, therefore, reject the second as well as the first division of the views 

 under discussion ; not only is the muscle not fed exclusively on proteid 

 material, but also its energy does not arise from an exclusively proteid 

 metabolism. 



Animal Heat. 



444. 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 shown 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 chemical changes, 

 which we may speak of generally as those of oxidation, undergone not by 

 any particular substances, but by the tissues at large. Wherever metab- 

 olism is going on, or, to be more exact, wherever destructive metabolism, 

 katabolism, 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 constructive metabolism, the anabolism of the body, 

 and in the smaller synthetic processes of which we spoke in dealing with urea 

 ( 403), heat is undoubtedly to a certain extent being absorbed and ren- 



