METABOLISM 7 



and (2) the influence of muscular labour. As regards 

 growth, physiology teaches that it can only take place 

 when protein is supplied in excess of the actual needs of 

 the body as measured by the output of nitrogen. In the 

 earlier period of life, therefore, a luxus consumption of 

 protein is actually necessary, a point which must be 

 borne in mind in constructing dietaries for the young. 

 On the other hand, experiment has also shown that, 

 once growth is completed, it is exceedingly difficult to 

 bring about a storage of protein in the body, except 

 when the muscles are being much exercised.* So soon, 

 moreover, as the period of exercise ceases the protein 

 gained is rapidly lost again. One cannot hope, there- 

 fore, to make an individual muscularly strong merely by 

 supplying him with protein ; exercise must always be 

 taken at the same time. Convalescence from acute 

 disease is another event which permits of protein being 

 stored up, for in such circumstances the waste of 

 tissue which has taken place is rapidly made good by 

 retaining protein in the body, even although the amount 

 supplied in the diet be not very great. During preg- 

 nancy, too, protein is retained in order to take part both 

 in the formation of the foetus and in the hypertrophy of 

 the uterus and mammae. 



As regards the influence of muscular labour, experi- 

 ment has clearly shown that the waste of tissue which 

 it causes is not really great, and there is therefore 1:0 

 physiological reason for the consumption of an excessive 



* The gain of protein in a fully-grown animal is attained by an 

 hypertrophy of cells already existing, not by the development of 

 new cells. 



