240 OF FOOD AND THE DIGESTIVE PKOCESS. 



and thus giving off as much heat as if the same materials were burned 

 in a furnace. And it has been further shown, that the immediate cause 

 of death in a warm-blooded animal, from which food has been entirely 

 withheld, is the inability any longer to sustain the temperature, which 

 is requisite for the performance of its vital operations ( 117). Hence 

 we see the necessity for a constant supply of aliment, in the case of 

 warm-blooded animals, for this purpose alone : and the demand will 

 be chiefly regulated by the external temperature. When the heat is 

 rapidly carried off from the surface, by the chilling influence of the 

 surrounding air, a much greater amount of carbon and hydrogen must 

 be consumed within the body, to maintain its proper heatj than when 

 the air is nearly as warm as the body itself; so that a diet which is 

 appropriate to the former circumstances, is superfluous and injurious 

 in the latter ; and the food which is amply sufficient in a warm climate, 

 is utterly destitute of power to enable the body to resist the influence 

 of severe cold. This is a fact continually experienced ; both in the 

 ordinary recurrence of changes of temperature in our own climate; 

 and, still more remarkably, when the same individual is subjected to 

 the extremes of heat and cold, in successively visiting the tropical and 

 frigid zones. 



414. Thus we find that in the Animal body, aliment is ordinarily 

 required for four different purposes. First, for the original construc- 

 tion or building-up of the organism. Second, to supply the loss occa- 

 sioned by its continual decay, even when in a state of repose. Third, 

 to compensate for the waste occasioned by the active exercise of the 

 nervous and muscular systems. And Fourth, to supply the materials 

 Tor the heat-producing process, by which the temperature of the body 

 is kept up. The amount required for these several purposes will vary 

 according to the conditions of the body, as regards exercise or repose, 

 and external heat or cold. It is also subject to great variation with 

 difference of age. . During the period of growth, it might be anticipated 

 that a larger supply of food would be required, than when the full 

 stature has been attained ; but a very small daily addition would suffice 

 in the case of a child or youth, to produce the entire increase of a whole 

 year. Yet every one knows that the child requires much more food 

 than the adult, in proportion to his comparative bulk. This results from 

 the much more rapid change in the constituents of his fabric ; which is 

 evident from the large proportional amount of his excretions, from the 

 quickness with which the effects of illness or of deficiency of food mani- 

 fest themselves in the diminution of the bulk and firmness of the body, 

 from the short duration of life when food is altogether withheld, and 

 from the readiness with which losses of substance by disease or injury 

 are repaired, when the nutritive processes are restored to their full 

 activity. The converse of all this holds good in the aged person. The 

 excretions diminish in amount, the want of food may be sustained for a 

 longer period, losses of substance are but slowly repaired, and everything 

 indicates that the interstitial changes are performed with comparative 

 slowness ; and, accordingly, the demand for food is far less in propor- 

 tion to the bulk of the body than it is in the adult, and may be even 

 absolutely less than in the child of a fourth of its weight. 



