282 SENESCENCE AND REJUVENESCENCE 



unable to recover its full capacity for growth. Stunting in man and 

 the mammals is undoubtedly due in large measure to subnormal 

 skeletal growth, and while the effect of long-continued underfeeding 

 on the physiological condition of the skeletal tissues is not known, 

 the facts suggest that the usual relation between senescence and 

 growth is altered. In other words, the cells which give rise to the 

 skeletal substance probably undergo some degree of senescence 

 during underfeeding without being able to produce as much skeletal 

 substance as under continuous good nutritive conditions, conse- 

 quently their rate of metabolism is lower and they are less capable 

 of producing skeletal substance after such a period than the cells 

 of an individual of the same size which has been continuously well 

 fed. The skeleton of the individual which has been subjected 

 to underfeeding for a sufficiently long time will therefore cease to 

 grow, even under good nutritive conditions, at a smaller size than 

 that of the continuously well-fed individual, and very probably 

 the same is true to a greater or less extent in other tissues. In the 

 underfed animal the proportion of more stable to less stable com- 

 ponents of the tissues must increase more rapidly than where nutri- 

 tion is sufficient for all requirements, for in the former case the less 

 stable components must break down to a larger extent than in the 

 latter. In the absence of sufficient food these substances must 

 serve to a larger extent as a source of energy or for the synthesis 

 of the more stable components than where sufficient nutritive sub- 

 stance is available. Consequently the substitution of more stable 

 for less stable substances in the tissues goes on during the period 

 of underfeeding, but with less than the usual amount of growth 

 because the less stable substances are present as structural com- 

 ponents in smaller proportion than under the usual conditions. 

 After a long period of underfeeding the tissues are physiologically 

 older and therefore less capable of growth, even when nutrition 

 is present in excess, than in the continuously well-fed animal of the 

 same size. According to this conception, senescence in the higher 

 animals and man may proceed to some extent even when httle or 

 no growth occurs, because the body substance is gradually trans- 

 formed to a greater or less extent from more active to more stable 

 conditions. 



