152 OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



the sustentation of its activity. The human infant at first possesses but a feeble 

 heat-producing power; and the lower the temperature to which it is exposed, 

 the more does it depend upon some external source of warmth. As its digestive 

 capacity improves, however, and it can appropriate an adequate supply of food, 

 its calorific power augments; for its rapid circulation and active respiration 

 enable the combustive process to be performed with an energy greatly surpassing 

 that which is displayed in later life, as is shown in the quantity of carbonic acid 

 thrown off. And thus, as it is from its food that the organism derives not 

 merely its materials but its vital force, and as the expenditure of both is pecu- 

 liarly rapid, it comes to pass that the dependence of life upon a continual supply 

 of food is far more close at this period than subsequently; so that when children 

 and adults are subjected at the same time to complete or partial starvation, the 

 former succumb much earlier than the latter. 



132. The period of adult age is marked by an increase alike in the nervo- 

 muscular power of the body, and in its general vigor and endurance ; the aug- 

 mentation of the latter being most strongly displayed in the activity of the 

 generative function. Still it cannot be said that its vital force is on the whole 

 increased in proportion to its bulk ; for the formative power is decidedly dimin- 

 ished. The production of new tissue is now for the most part limited to the 

 replacement of that which has become effete by use; there is no longer a capa- 

 city for the production of new organs, and comparatively little for the augmen- 

 tation of those already existing; the increase of the uterine and mammary 

 structures, during the period of gestation, being the most important examples of 

 formative power, and these presenting themselves in the sex in which there is 

 least of nervo-muscular activity and of general vigor. We should infer, then, 

 that the " germinal capacity" is now on the decline; and this further appears 

 from the inferior energy and completeness with which the reparative processes 

 are performed, as compared with the mode in which they are executed during 

 the period of growth. Moreover, the ordinary rate of waste or degeneration of 

 tissue is now much less rapid than during the period of growth; for we have 

 seen that decay and removal, in the latter case, are among the very conditions 

 of increase ; whilst in the former, they proceed, for the most part, only from 

 the expenditure of the vital powers of the tissues, consequent upon their func- 

 tional activity. The whole nisus of development, in fact, during this period, 

 appears to be directed towards the maintenance of the organism in the state 

 which it had acquired at its commencement, by the regeneration of its tissues as 

 fast as they undergo disintegration, and by the renovation of its vital force in 

 proportion as this is expended. There is consequently a less demand for aliment- 

 ary material, than during the previous periods (allowance being made for the 

 augmented bulk of the body) ; the proportional amount of heat produced (as 

 indicated by the carbonic acid exhaled) is also less ; and the dependence of life 

 upon a constant supply of aliment is far less close. 



133. The decline of life exhibits a much more obvious diminution of the 

 whole vital power of the organism ; for not only is its formative activity now 

 greatly reduced, but its nervo-muscular energy and general vigor progressively 

 diminish, and its generative power declines or ceases entirely. Of this diminu- 

 tion in formative power, we have evidence in the entire absence of any attempt 

 at new development, in the less perfect and more tedious manner in which the 

 losses of substance occasioned by disease or injury are recovered from, and in 

 the gradual degeneration of the organism in general. The tissues which are 

 rendered effete by their functional activity are not any longer replaced in their 

 normal completeness : for, either the quantity of new tissue is inadequate, so that 

 the bulk of the organs is obviously reduced; or their quality is rendered imper- 

 fect, by the production of structures in various phases of degeneration (especially 

 the fatty), in place of those which had been previously developed in the fullest 



