SENESCENCE IN HIGHER ANIMALS AND MAN 281 



if it occurs anywhere, would be limited to the more stable tissues or 

 substances of the body, while the less stable would undergo more 

 or less reduction, for in the one case the losses from breakdown are 

 slight and are more than balanced, while in the other they are 

 greater and are not balanced and the products of breakdown of 

 the less stable tissues take part to a greater or less extent in the 

 upbuilding of the more stable. The organic structural substance 

 of the skeleton is scarcely to be regarded as living; it is rather of 

 the nature of a secretion, and after its formation it takes but little 

 part in metabolism, except when altered functional conditions 

 determine a change in bone structure. Consequently in underfed 

 animals there is little loss of skeletal substance, and every addition 

 counts for growth. Skeletal growth may therefore continue while 

 reduction is going on in various other organs, the products of 

 breakdown of the latter serving to build up the more stable sub- 

 stance of the former. 



As regards the nervous system, conditions are somewhat 

 similar. The nervous system is certainly one of the most stable, 

 perhaps the most stable living tissue in the body. Its cells persist 

 throughout life, and dedifferentiation of nerve cells is not known to 

 occur in vertebrates. The losses of the nervous system during 

 starvation are relatively slight, and in underfed animals it main- 

 tains its weight or grows at the expense of the less stable tissue, 

 as the products of their breakdown are synthesized into more 

 stable forms in the nervous system, and so become more permanent 

 constituents of the structural substratum of the bod>'. This 

 stability of the nervous system is not, however, like that of the 

 skeleton, the stability of a dead secretory substance, but is the 

 stability of a living protoplasm and is undoubtedly associated with 

 the high metabolic rate in the nervous system. 



The result of return to a normal diet after a period of insufiicient 

 nutrition apparently depends in part on the length of that period. 

 It has been clearly demonstrated that in man as well as in animals 

 the retarding effect upon growth of even a considerable period of 

 insuflfiicient nutrition may be compensated later on a normal diet. 

 But it is also true that a sufficiently long period of underfeeding 

 may result in permanent "stunting," the body apparently being 



