VARIATIONS OF NUTRITION WITH AGE. 349 



Respiratory organs, together with other parts of the Excretory appa- 

 ratus, are so completely evolved, as to be able to separate the effete 

 matter, and to cast it out of the system with an energy equivalent to 

 that of the organs, by which new matter is introduced and appropriated. 

 On the other hand, the Brain, although of larger comparative size at 

 birth, than at any subsequent period of life, is but very imperfectly 

 developed ; for its structure is not yet so far completed, as to prepare 

 it for a state of high functional activity. In fact it would seem as if 

 the use of the organ, as called forth by the new circumstances in which 

 the infant is placed as soon as it comes into the world, is essential to 

 its complete development ; and the same may be said of the Muscular 

 system. 



625. During the whole period of infancy and childhood, the current 

 of nutrition seems peculiarly directed towards the brain ; for though 

 its size does not continue to increase, in proportion with that of the 

 remainder of the body, its structure is evidently being rendered more 

 perfect, and its functional activity is excited with remarkable facility. 

 Hence it is peculiarly liable to be acted on by various causes which 

 may produce disease ; and the operation of remedies, which specially 

 affect that organ, is far more powerful than at any other period of life. 

 Thus, whilst, a child will bear a fourth, or even a third of the dose of a 

 purgative adequate for an adult, it is strongly affected by an eighth, or 

 even a twelfth of the dose of a narcotic or a stimulant that would be 

 required to produce a corresponding effect in middle life. This peculiar 

 impressibility of the nervous system, resulting from the activity of the 

 nutrient processes which are taking place in it, manifests itself also in 

 other ways ; thus children are peculiarly liable to have its powers de- 

 pressed by any sudden shock, such as a blow, or an extensive burn or 

 laceration ; whilst, on the other hand, if the depression be not fatal, 

 they recover from its effects much more speedily than an adult would 

 do from a similar condition. 



626. During the periods of youth and adolescence, the chief energy 

 of development (except in regard to the generative system, already 

 noticed,) appears to be directed towards the Muscular apparatus ; which 

 then increases in vigour, in a degree which surpasses its increase of 

 size ; and the circulating and respiratory organs, upon whose energetic 

 action there is then a corresponding demand, are peculiarly liable to 

 disturbance of function, inducing disease in themselves or in. other parts. 

 The maladies of this period are for the most part of a sthenic or inflam- 

 matory character ; resulting, as we shall presently see, from the exces- 

 sive activity of the assimilating processes, which are disposed to produce 

 more fibrine than the wants of the body require. Or if, on the other 

 hand, there be an imperfect elaboration of the nutrient materials, as 

 happens in the tubercular diathesis, its effects are peculiarly liable to 

 manifest themselves at this period, when the demand for nutritive matter 

 is greatly augmented by the activity of the muscular system. 



627. In adult age, there should be such a balance of all the functions, 

 arising from the due development and proper use of each organ, as may 

 preserve the body in the state of health and vigour, without any marked 

 change in the relative dimensions of its different parts, through a long 



