GROWTH 



383 



with the supply in the feed (402), as indeed is really implied 

 in the conception of maturity. By a mature animal is meant 

 one which has completed its growth, and growth consists essen- 

 tially in an increase of the nitrogenous structural elements 

 of the body. Obviously, therefore, if the capacity for growth 

 has been exhausted, no material storage of protein can occur 

 and an excess of this material above the maintenance require- 

 ment will serve chiefly or wholly as a source of energy to the 

 organism. 



With the young animal the case is different. Its rapidly 

 growing cells and tissues demand a liberal supply of protein, 

 and if this is afforded by the feed it is largely utilized to build 

 up tissue instead of undergoing nitrogen cleavage. Conse- 

 quently, other things being equal, a much larger percentage of 

 the feed protein is retained in the body. 



The investigations whose results have been considered on previous 

 pages (463) , especially those upon the younger animals, afford striking 

 illustrations of this fact, Soxhlet's experiments upon calves being the 

 earliest and most familiar. Their results are summarized in the fol- 

 lowing table, the feed consisting of fresh whole milk ad libitum. 



TABLE 82. PERCENTAGE OF FEED PROTEIN RETAINED SOXHLET 



More recent and even more striking illustrations of the same fact 

 are afforded by Fingerling's experiments. Thus, in one instance 

 a calf averaging 9 days old received whole milk and in a succeed- 

 ing period milk with the addition of butter fat and lactose, and 

 retained the percentages of digested protein shown in the following 

 table. 



