GROWTH AND SENESCENCE 



4i 



tween the phenomenon under consideration and other phe- 

 nomena, but they do not give the ultimate nature or meaning of 

 the phenomenon. 



In preparing this set of curves of changes with age we have 

 kept in mind the idea, not merely of giving a quantitative pic- 

 ture of the age changes, but also of discovering a law of growth 

 which should indicate the similarity between the course of 

 growth and the course of physico-chemical changes outside of 

 the body. This attitude of comparing growth to a physico- 

 chemical process seems to be legitimate in view of the fact 

 that growth is by definition a physico-chemical process — cer- 

 tain substances typical of foodstuffs are obviously converted 

 during the course of growth and senescence into other sub- 

 stances typical of the body tissues and fluids. Of course, growth 

 is not merely a physico-chemical reaction, but it is conceivable 

 that in spite of all the complexities of growth there may be one 

 reaction which dominates the course of growth during each 

 phase of growth. The fact that the secretion of one gland such 

 as the thyroid or pituitary can dominate the course of growth is 

 good substantiating evidence of the theory that the course of 

 growth may be controlled by the slowest or limiting reaction. 

 That one reaction may, and usually does, limit the course of a 

 complex chemical process is a well-known principle in chemistry. 



II. The Curve Representing the Life Cycle 



The purpose of this section 

 is to attempt to derive a law of 

 changes due to age including 

 the whole of the life cycle. 

 Figure 21 shows the changing 

 resistance with age against fa- 

 tal pneumonia. We see the 

 curve rising up to the age of 

 thirteen years, that is, up to 



I 6 



H 6 



"SI 



HL 



e s2 



1 



\baO 10 20 30 40 50 60 70 

 Age 



Figure 21. Age changes in resistance to 



fatal pneumonia. The smooth curve repre- 



. sents the formula on the chart; the circles 



the age or puberty. The resist- are observed values. 



