616 METABOLISM 



tions with various food mixtures must be conducted over long periods 

 of time. The nutritive values of the common cereals added to a stand- 

 ard diet that had brought the animals (rats) to the threshold of death, 

 were found to be as follows: With cornmeal there was immediate recov- 

 ery and rapid growth, both of which were also secured in considerable 

 degree by wheat embryo and entire wheat kernel ; with rye and oats, on 

 the other hand, there was little if any improvement. 



Much work is, of course, yet to be done before we can determine the 

 exact role which each unit plays in the physiological development of 

 young animals. To sum up what we already know, it may be said that 

 glycocoll is not essential, since it can be manufactured by the animal 

 itself; that tryptophane is essential for maintenance, probably because 

 it is required for the production of certain essential hormones, for the 

 make-up of which in its absence other tissues must become disintegrated, 

 leading therefore to a diminution in body weight; and that lysine ap- 

 pears to be essential for growth. Tissues can be maintained without 

 lysine, but they can not grow. That the young rats in the experiments 

 of Mendel grew normally while living on milk supplied by the stunted 

 mother indicates that the requisite lysine must have been produced in 

 the mother's body. 



In the application of the foregoing principles to human dietetics, it 

 is undoubtedly safe to follow Bayliss's advice to take care of the calo- 

 |ries and allow the proteins to take care of themselves. 11 For example, 

 in the case of milk the deficiency of cystine in its chief protein, casein, 

 is corrected by the presence of lactalbumin, which, though present in 

 only small amounts, contains sufficient quantities of this amino acid to 

 meet the demands of the growing tissue. 



These observations on maintenance and growth suggest very interest- 

 ing applications in connection with the growth of tumors. Is it possible 

 that we might retard the growth of tumors by a diet that was insufficient 

 for growth while sufficient for maintenance. In an experiment devised 

 to test this proposition mice were fed on a diet of starch, lard, lactose 

 and gluten on which they could merely maintain existence but failed 

 to grow. Some of these rats were inoculated with a rapidly growing 

 tumor at the same time as another batch of mice kept on normal diet, and 

 it was found that the tumor grew much more slowly in the stunted mice 

 than in the others. One mouse, for example, on the restricted diet had 

 a scarcely visible tumor 52 days after the inoculation. When this mouse, 

 however, was placed on a normal diet of bread, milk, etc., the tumor 

 immediately began to grow at a very great rate. 13 Too much importance 

 should not be placed on this experiment. 



We shall now pass on to consider some of the factors besides the pro- 

 tein content which have an important bearing on dietetic efficiency. 



