CONDITIONS WHICH INFLUENCE NUTRITION. 443 



the nutritive forces. This does not imply that nutrition is effected through 

 the influence of the nerves, but it is the fact that certain nerves, by regulat- 

 ing the supply of blood, and perhaps by other influences, are capable of 

 modifying the nutrition of parts to a very considerable extent. 



As regards the influence of exercise upon the development of parts, it has 

 been shown that this is not only desirable but indispensable ; and the proper 

 performance of the offices of nearly all parts involves the action of the nerv- 

 ous system. It is true that the separate parts of the organism and the organ- 

 ism as a whole have a limited existence ; but it is not true that the change 

 of nitrogenized substances into effete matters a process that is increased in 

 activity by physiological exercise consumes, so to speak, a definite amount 

 of the limited life of the parts. Physiological exercise increases disassimila- 

 tion, but it also increases the activity of nutrition and favors development. 

 It is often said that bodily or mental effort is made always at the expense of 

 a definite amount of vitality and matter consumed. This is partly true, but 

 mainly false. Work involves change into effete matter ; but when restricted 

 within physiological limits, it engenders a corresponding activity of nutri- 

 tion, assuming, of course, that the supply from without be sufficient. Other 

 things being equal, a man would live longer under a system of physiological 

 exercise of every part than if he made the least effort possible. It is, indeed, 

 only by such use of parts, that they can undergo proper development and 

 become the seat of normal nutrition. Notwithstanding all these facts', life 

 is self-limited. Organic substances are constantly undergoing transforma- 

 tion. In the living body, their metabolism is unceasing ; and after they are 

 removed from what are termed vital conditions, they change, first losing 

 excitability, and afterward decomposing into matters which, like the products 

 of their disassimilation, are destined to be appropriated by the vegetable 

 kingdom. Nutrition sufficient to supply the physiological decay of parts 

 can not continue indefinitely. The forces in the fecundated ovum lead it 

 through a process of development that requires, in the human subject, more 

 than twenty years for its completion ; and when development ceases, no one 

 can say why it becomes arrested, nor can any sufficient reason be given why, 

 with an adequate and appropriate supply of material, a man should not grow 

 indefinitely. When the being is fully developed, and during what is known 

 as adult life, the supply seems to be about equal to the waste ; but after this, 

 nutrition gradually becomes deficient, and the deposition of new matter in 

 progressive old age becomes more and more inadequate to supply the place 

 of the nitrogenized substance. There may be at this time, as an exception, 

 a considerable deposition of fat ; but the nitrogenized matter is always de- 

 ficient, and the proportion of inorganic matter combined with it is increased. 



There can be little if any doubt that the properties which involve the 

 regeneration or nutrition of parts reside in the organic nitrogenized sub- 

 stance, the inorganic matter being passive, or having purely physical uses. 

 If, therefore, as age advances, the organic matter be gradually losing the 

 power of completely regenerating its substance, and if its proportion be pro- 

 gressively diminishing while the inorganic matter is increasing in quantity. 



