CHARACTERISTICS OF DIFFERENT AGES. 1015 



CHAPTER XX. 



OF THE MODES OF VITAL ACTIVITY CHARACTERISTIC OF 



DIFFERENT AGES. 



860. ALTHOUGH from the time when the Human being comes into the 

 world, to the final cessation of his corporeal existence, the various functional 

 operations of Organic life are carried on with ceaseless activity, whilst those 

 of Animal life are only suspended by the intervals of repose which are 

 needed for the renovation of their organs, yet there are very marked differ- 

 ences, not only in the degree of their united activity, but also in the relative 

 degrees of energy which they severally manifest, at different epochs. These 

 differences, taken in connection with the modifications in the size and con- 

 formation of the body with which they are in relation, mark out the whole 

 term of life into the various " Ages," which are commonly recognized as 

 seven namely, Infancy, Childhood, Youth, Adolescence, Manhood, De- 

 cline, and Senility. For Physiological purposes, however, a less minute 

 subdivision is equally or perhaps more appropriate ; namely, the three 

 great periods of Growth and Development, of Maturity, and of Decline. The 

 first comprehends the whole of that series of operations, by which the germ 

 evolves itself at the expense of the nutriment which it appropriates from 

 external sources, into the complete organism, possessed not merely of its full 

 dimensions, but of its highest capacity for every kind of functional activity ; 

 this includes, therefore, the epochs of Embryonic life, Infancy, Childhood, 

 Youth, and Adolescence, all of which are characterized by an excess of the 

 constructive over the destructive changes taking place in the organism. The 

 second period ranges over the whole term of Manhood, in which the organ- 

 ism, having attained its complete development, is brought into vigorous and 

 sustained activity ; and in which it is maintained in a condition fitted for 

 such activity, by the equilibrium which subsists between the operations of 

 red integration and of disintegration. The third period commences with the 

 incipient failure of the bodily powers, consequent upon the diminished ac- 

 tivity of the constructive powers, as compared with that of the changes which 

 involve degeneration and decay; this diminution begins to manifest itself 

 during the latter part of Middle Life, before Old Age can properly be said 

 to commence ; and it continues in an increasing ratio through the whole 

 "decline of life," until the reparative powers being exhausted, Death super- 

 venes as the necessary termination of that long succession of phenomena 

 of which Life consists. 



861. Although the organization of the body at each epoch may be truly 

 said to be the resultant of all the material changes which it has undergone 

 during the preceding periods, yet it is scarcely possible to take an enlarged 

 view of the case, without perceiving that we must look for the cause of this 

 succession in those dynamical conditions, the presence of which is the distin- 

 guishing attribute of living structures. Every constructive act, whether this 

 consist in Growth ( 331) or in Development ( 332), not merely requires 

 materials for the new tissue produced, but depends upon the active operation 

 of a formative poiver, without whose agency these materials would remain 

 unorganized. In our examination into the source of the formative power 

 which we see thus operating in every individual organism (chap, i), we 

 found it traceable to the Physical Forces to which it is subjected (Heat being 



