SOURCES OF DEMAND FOR FOOD. 239 



destined to serve some other purpose in the system, or that has already 

 answered it ; the remainder (that of which the solid walls are composed) 

 being furnished by the nutritive materials of the blood, and being 

 henceforth altogether lost to it. Thus every act of Nutrition involves 

 a waste or decay of Organized tissue. 



412. We may observe a marked difference, however, between the 

 amount of aliment required, and the amount of waste occasioned, by the 

 simple exercise of the nutritive or vegetative functions in the building- 

 up and maintenance of the animal body, and that which results from 

 the exercise of the animal functions. The former are carried on, with 

 scarcely any intermixture of the latter, during foetal life. The aliment, 

 in a state of preparation, is introduced into the foetal vessels ; and is 

 conveyed by them into the various parts of the structure, which are 

 developed at its expense. The amount of waste is then very trifling, as 

 we may judge by the small amount of excretory matter, the product of 

 the action of the liver and kidneys, which has accumulated at the time 

 of birth ; although these organs have attained a sufficient development 

 to act with energy when, called upon to do so. But as soon as the 

 movements of the body begin to take place with activity, the waste in- 

 creases greatly; and we even observe this immediately after birth, when 

 a large part of the time is still passed in sleep, but when the actions of 

 respiration involve a constant employment of muscular power. In the 

 state of profound sleep, at subsequent periods of life, the vegetative 

 functions are performed, with no other exercise of the animal powers, 

 than is requisite to sustain them ; and we observe that the waste, and 

 the demand for food, are then diminished to a very low point. This is 

 well seen in many animals, which lead a life of great activity during 

 the warmer parts of the year, but which pass the winter in a state of 

 profound sleep, without, however, any considerable reduction of tempe- 

 rature ; the demand for food, instead of being frequent, is only felt at 

 long intervals, and the excretions are much reduced in amount. And 

 those animals which become completely inert, either by the influence of 

 cold, or by the drying-up of their tissues, do not suffer from the pro- 

 longed deprivation of food ; because not only are their animal functions 

 suspended, but their nutritive operations also are in complete abey- 

 ance ; and the continual decomposition of their tissues, which would 

 otherwise be taking place, is checked by the cold or desiccation ; so that 

 the whole series of changes which goes on in their active condition, is 

 completely at a stand. 



413. But there is another most important cause of demand for food, 

 amongst the higher Animals, which does not exist either amongst the 

 lower Animals, or in the Vegetable kingdom. We have seen (CHAP. II.) 

 that Mammals and Birds, and to a certain extent Insects also, are 

 able to sustain the heat of their bodies at a fixed standard, and thus 

 to become independent of variations in external temperature. This 

 they are enabled to do, as will be explained hereafter, by a process 

 strictly analogous to ordinary combustion ; the carbon and hydrogen 

 which are directly supplied by their food, or which have been employed 

 for a time in the composition of the living tissues and are then set free, 

 being made to unite with oxygen introduced by the respiratory process, 



