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 pabulwm. Con- 
sequently the current idea regarding the nutritive value of 
jellues 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 the other ; and it would seem as if this restlessness 
were a natural instinct, impelling them to use muscular exer- 
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