480 OF FOOD, AND THE DIGESTIVE PROCESS. 



so that it ceases to be fit to form part of the body, and is cast out by the va- 

 rious processes of excretion. The same is the case in regard to the Muscu- 

 lar substance ; the waste of which is conformable to the use made of it. The 

 demand for the materials of reparation will follow the same proportion ; and 

 as the preparation of these materials can only be effected by the agency of 

 the Vegetative or nutritive functions, the rate at which these are performed 

 will be greatly influenced by the activity of the Animal functions. Hence 

 we see the necessity of regulating the supply of food, in accordance with the 

 state of the latter ; since a diet which would be superfluous and injurious to 

 an individual of inert habits, is suitable and beneficial to one who is leading a 

 life of continual exertion. This difference manifests itself remarkably in the 

 contrast between Animals of different tribes, whose natural instincts lead them 

 to different modes of life. The Birds of most active flight, and the Mammals 

 which are required to put forth the greatest efforts to obtain their food, need 

 the largest and most constant supplies of nutriment ; but even the least active 

 of these classes stand in remarkable contrast with the inert Reptiles, whose 

 slow and feeble movements are attended with so little waste, that they can 

 sustain life for weeks and even months, with little or no diminution of their 

 usual activity, without a fresh supply of food.* 



634. Finally, there is a 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, at least to any great degree. In the classes of 

 Mammals and Birds, and in that of Insects also, we find a capability of sus- 

 taining the heat of the body at a fixed standard ; which is usually far above 

 that of the surrounding medium. 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 their living tissues and 

 then set free, being made to combine with Oxygen introduced by the respira- 

 tory process, and thus giving out the same heat, as if the same materials were 

 burned in a furnace. It will be hereafter shown that the immediate cause of 

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

 held, is the inability any longer to sustain the temperature which is requisite for 

 the performance of its vital operations (Chap. XVI., Sect. 2). 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 regulated by the 

 external temperature. When the heat is rapidly carried off from the surface 

 by the chilling influence of the surrounding air or water, a much greater 

 amount of Carbon and Hydrogen must be consumed within the body, to main- 

 tain its proper heat, than when the medium is nearly as warm as the body 

 itself; so that a diet, which is appropriate in 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 it to resist the influ- 

 ence of severe cold. Substances rich in carbon and hydrogen, and having 

 little or no oxygen, afford the most efficient heat-sustaining materials ; but it 

 is an essential condition of their due action, that they should be of a kind 

 that renders them capable of being reduced by the solvent action of the sto- 

 mach, and of being absorbed into the system. 



635. The demand for food is increased by any cause which creates an 

 unusual drain or waste in the system. Thus an extensive suppurating action 



* The materials which are ret|uirecl for the reparation of the Muscular tissue, are chiefly 

 of a fibrinous nature; those employed for the renovation of the Nervous substance, would 

 seem to be iatty matter with I'lm-phorus. But from the peculiar composition of the fatty 

 matters of the Nervous substanoe (especially the presence of Azote in them), it seems quite 

 uncertain from which of the constituents of the i'<>d they are really formed. 



