154 SOURCES OF DEMAND FOR ALIMENT. 



is strong reason to believe that these, like the albuminous, 

 are formed at the expense of the albuminous matter of the 

 blood, and that gelatin thus introduced undergoes a rapid 

 decomposition, yielding up a considerable part of its carbon 

 and hydrogen to the combustive process, which is the only 

 function to which it affords any substantial pabulum. Con- 

 sequently the current idea regarding the nutritive value of 

 jellies of various kinds, has little or no real foundation. 



160. It has been already stated ( 68) that all the living 

 tissues of the body are continually undergoing a sort of death 

 and decay ; and that they do this the more rapidly, in pro- 

 portion as they are called upon for the discharge of their 

 functions. The need of material capable of replacing that 

 which has been lost, is consequently the chief source of the 

 constant demand for aliment. Even in young, actively 

 growing animals, the quantity required for the increase of 

 their bodies constitutes but a very small proportion of that 

 which is taken in ; of the remainder, a part is at once re- 

 jected as indigestible ; and the rest is appropriated to the 

 repair of the waste which is continually going on. This waste 

 is much greater in young animals than in adults ; for all their 

 vital processes are more actively and energetically performed : 

 their movements are quicker in proportion to their size ; and 

 injuries are more speedily repaired. To remove the products 

 of this decomposition is the special object of the various pro- 

 cesses of excretion ; and among these, the respiration, by 

 which a large quantity of carbon and hydrogen is carried 

 off in the form of carbonic acid and water, is of the most 

 constant importance, on account of the heat which it thus 

 enables the animal body to maintain. This temperature, in 

 Carnivorous animals, appears to be sufficiently kept up by 

 the combustion of the carbon and hydrogen set free by the 

 decay (or metamorphosis, as it may be termed) of their tis- 

 sues ; but this combustion goes on with much more rapidity, 

 in consequence of their almost unceasing activity, than it does 

 in the Herbivorous animals, which lead comparatively inac- 

 tive lives. Every one who has visited a menagerie must have 

 noticed the continual restlessness of the Tigers, Leopards, 

 Hyenas, &c., which keep pacing from one end of their narrow 

 cages to tjie other ; and it would seem as if this restlessness 

 were a natural instinct, impelling them to use muscular exer- 



