592 THE ANIMAL BODY AS A MACHINE 



Tissues in young animals, secondly the larger proportion of surface to 

 volume involving a greater Radiation of Heat than in the adult, and 

 thirdly the energy absorbed in the building up and retention of new 

 tissue. In older children we must add to these the incessant Muscular 

 Activity which characterizes a healthy child. Taylor states that a 

 resting boy of ten years should have a metabolism of about 40 calories 

 per kilo per day, but when engaged in play the diet of the child may 

 have to be as high as 100 calories per kilo per day to maintain calorific 

 equilibrium. "The diet of a child must, therefore, cover the basal 

 metabolism, the natural increment of growth, and the enormous output 

 for physical exercise. It is the inability to judge these fractions 

 correctly that is responsible for so much underfeeding of children. 

 There are furthermore the additional deprivations so often inflicted 

 on children by the application of fad-notions of diet. The relative 

 caloric input of a normal child leading an outdoor life is to be compared 

 to that of a man at heaviest physical work. Protein in excess is not 

 needed, that is clear; but total calories are needed, in the form of sugar 

 and fat." The craving of healthy children for sugar is therefore the 

 expression of a normal and healthy need arising from the high con- 

 sumption of glucose by the Muscular Tissues. It should be satisfied 

 by a discreet allowance of sugar and an abundant allowance of poly- 

 saccharides. 



THE ENERGY-EQUIVALENT OF GROWTH. 



The storing-up of tissue-substance which is possessed of a definite 

 calorific value, necessarily results in the retention by the growing 

 animal of a proportion of the energy- value of its food, and, furthermore, 

 a considerable proportion of the , heat- value of the diet, varying with 

 age and the rate of growth, is additionally consumed and dissipated in 

 performing the work of storage. This is doubtless attributable to the 

 fact that at all stages of growth, as at all stages of any chemical trans- 

 formation, the forward and reverse reactions are proceeding side by 

 side. In growth the products of the reverse reaction (tissue-degrada- 

 tion) participate in a side-reaction (Exogenous Metabolism) and are thus 

 partially consumed and their energy- value dissipated in the form of 

 heat, mechanical work, and the energy-values of the excreta. Hence 

 we find that in a given species of animal, the slower the accretion of 

 tissue the greater the energy consumed per kilo of tissue built up, since 

 the reverse reaction in such a case is proceeding for a longer time. The 

 following are illustrative results obtained by Aron: 



GROWTH OF DOGS. 



Calories consumed 



Animal Calories consumed Increase in weight per gram of 



number. in fifty days. in fifty days. tissue-increase. 



B 19,950 1570 12.7 



C 13,925 1000 13.9 . 



VIII 9,500 780 16.4 



XII 10,750 838 15.6 



