GROWTH AND THE AGING PROCESS 957 



hip. Here again the truly "senescent" factors involved in the train of events proba- 

 bly were (i) the increasing feebleness which was a factor in the accidental fall and 

 (2) the increased brittleness of the bone (Cobb, 1952). On the basis of what he 

 considei's the processes which represent true senescence, Robertson describes the 

 latter as incompatible with growth. 



This means that in a given mass of protoplasm the two processes cannot be 

 occurring in the same location at the same time. Obviously, in a large organism 

 growth processes in individual cells, in tumor masses, or even in separate organs, 

 may be occurring although the organism as such is senescent, and on the down- 

 ward slope of the life curve. 



We may inquire now concerning the physical, chemical and biological charac- 

 teristics of the process of growth upon the one hand and of senescence upon the 

 other. While no one is more fundamental than the others, we may consider first the 

 physical aspects. 



If growth is an increase in ainount of living matter, is senescence a decrease? 

 Or is it simply the reaching of a plateau of quantity of protoplasm, the mere cessa- 

 tion of increase then setting in motion forces of change, not necessarily involving loss 

 of protoplasm but nevertheless leading inevitably onward to degeneration. Such a 

 process might well be compared to the cessation of flow of a rapidly coursing 

 stream as it settles first into a slowly running one and ends finally as a series of 

 stagnant pools or a patch of marshland. While the waters leap and sparkle among 

 the rocks, they are clean, pure, and "living". As the flow (growth) is slowed and 

 when it finally ceases altogether, new processes are initiated, the cleanliness and 

 purity are lost, and the waters become choked by foreign growths. While this 

 analogy should not be carried too far, it may correspond in not too inaccurate a 

 way to the dynamics of growth and senescence. 



There is, in fact, not a great deal of evidence of loss of living material as such in 

 the senescent organism. More important, the loss which occurs may well be 

 accounted for by degenerative changes brought about by cessation of growth, 

 not as an orderly process (such as that of growth itself), but rather as a series of 

 varied phenomena in different organs or tissues, very probably as the individual 

 reaction of those organs and tissues to the process of "stagnation". We shall return 

 to the consideration of these varied reactions in discussing the biological character- 

 istics of growth and senescence. 



Our comparison of the physical characteristics of these processes is hampered 

 to some extent by our lack of knowledge of the physical organization of protoplasm. 

 We do know enough, however, to describe protoplasm as a colloid. Many pene- 

 trating attempts have been made to compare the aging of protoplasm with the 

 aging of "other colloids" (Dhar, 1932; Ruzicka, 1922). If such a comparison 

 could be considered valid, it would be of great interest and importance, since it 

 probably would concern the really fundamental nature of the age process. The 

 difficulties are considerable here, however, since one iscomparing a rather artificial 

 colloidal system in a laboratory tube with the mysterious living system of the cell 

 which, while it may have ceased to "grow", still is dynamic in the sense of present- 

 ing an interchange of molecules between itself and its environment. There is prob- 

 ably a constant shifting of molecules within its body from nucleus to cytoplasm, 



Literature p. gyi 



