SENESCENCE IN HIGHER ANIMALS AND MAN 



277 



Kossel, E. Fischer, and Abderhalden and their students has played 

 a very important part, has opened up new fields of investigation. 

 It is now possible to attack the problem of nutrition and its rela- 

 tion to physiological condition, maintenance, development, and 

 growth, at least in the higher animals, with more exact and more 

 scientific methods than heretofore. Since we have become familiar 

 with the nature of the constituent substances 

 (Bausteine, i.e., building stones), the amino- 

 acids and certain other substances, which go to 

 make up the proteid molecule, and know more 

 or less exactly which of these substances are 

 present and in what proportions in various pro- 

 teids, we are able by feeding animals with 

 different proteins or with one or another of the 

 constituent substances to learn something of 

 the capacities of the animal for building up its 

 own specific proteid molecules and of the rela- 

 tions of each of the various nutritive substances 



Days 3 8 19 2838 55 80 io6| 180 270 



FIG. 112. Curve showing the decrease in rate of growth in female rabbits from 

 ;-; to 270 days after birth: the intervals indicated are the same as in Fig. in. From 

 Minot's tables (Minot, '08). 



to its various activities. Many difficulties still exist in connection 

 with these investigations: the methods of isolating the various 

 substances in pure form for feeding are in many cases far from 

 satisfactory, and it is often difficult to devise a food from the 

 isolated substances which the animal will eat in quantities suffi- 

 cient to supply the necessary energy. Moreover, the complexity 

 of metabolism and the impossibility of following the various steps 

 within the organism are serious obstacles. Notwithstanding all 

 these difficulties there can be no doubt that this method of inves- 

 tigation will throw light on various features of life heretofore 

 obscure. Any extended discussion of the results already attained 



