44 



GROWTH 



measured by her capacity to secrete milk. The milk is seen to 

 reach a maximum between seven and eight years. This is also 

 the age when the body weight is at a maximum. The milk flow 

 then steadily declines. 



These figures indicate that while the processes of growth and 

 senescence are probably simultaneous, and that while it is true 

 that "we begin to die the very moment we are born," there are, 

 nevertheless, two more or less distinct phases: a phase of growth 

 during which growth is predominant, and a phase of senescence 

 during which the process of senescence is predominant. 



In harmony with the idea expressed in the preceding section 

 that the course of growth and senescence may be limited by a 

 chemical process, the equation chosen to fit these curves is one 

 resembling in form, at least, a certain type of chemical process, 

 namely, two simultaneous consecutive monomolecular reactions. 



777. The Curve Representing the Course of Senescence 

 and Its Equation 



Having obtained a view of the life cycle as a whole, we may 

 now consider in greater detail that phase of the age curve dur- 

 ing which the process of senes- 

 cence predominates, and if pos- 

 sible, obtain an equation or 

 law relating the degree of se- 

 nescence with age. 



Figure 25 shows the course 

 of senescence in the domestic 

 fowl as measured by the de- 

 cline in egg production with 

 age and by the decline in 

 growth-sustaining powers of 

 its serum. The circles repre- 

 sent the observed values. The 

 egg production per year is seen 

 to decline from 158 in the first 



20 M 28 32 36 3fl 



fife 



Figure 25. The course of senescence in the 

 domestic fowl as measured by the decline 

 in egg production, and by the decline in 

 growth-sustaining powers of its serum. 

 Also the course of senescence in man as 

 measured by the decline in the speed of 

 healing wounds of a certain size with ad- 

 vancing age. (The age curves for healing of 

 wounds and growth of fibroblasts in serum 

 were constructed on the basis of data pub- 

 lished by Carrel and coworkers.) 



