OF EXCRETION AND SECRETION. 293 
the purposes to which it is destined, it is requisite that these 
products should be drawn-off from the current of the circula- 
tion, as constantly as they are received into it; and this is 
accomplished by the various procésses of Excretion, which are 
continually taking place in different parts of the body. The 
uninterrupted performance of these is even more essential to 
the maintenance of life, than is an uninterrupted supply of 
nutritive materials ; for an animal may continue to exist for 
some time without the latter, but it speedily dies if either 
of the more important excretions be checked. We have a 
striking instance of this in the case of the Respiration, which 
may be regarded as a true function of Excretion, having for 
its object to set free Carbonic acid from the blood in a gaseous 
form,—thereby contributing to the introduction of Oxygen 
into the blood, for the various important actions to which 
that element is subservient, especially the maintenance of 
_ Animal Heat. (Chap. rx.) The effects of the suspension of 
the respiratory process, even for a few minutes, in a warm- 
blooded animal, have been shown (§ 338) to be certainly and 
speedily fatal ; and they are as certainly fatal in the end in 
cold-blooded animals, though a longer time is required to 
_ produce them. 
346. The products of excretion are the same, as to their 
_ essential characters at least, through the whole Animal king- 
dom ; and for this it is not difficult to find a reason. It will 
be remembered that the ultimate elements of the Animal 
tissues are four in number: oxygen, hydrogen, carbon, and 
nitrogen ; and that the materials which make up the chief 
part of the fabric of different classes of animals—albumen, 
gelatin, fatty matter, &c.—contain these elements united in 
constant proportions, from whatever source we obtain them. 
Hence we should expect to find the products of their decom- 
position also the same; and this is, for the most part, the 
case. Of these four ingredients, oxygen can never be said 
(in the healthy state at least) to be superfluous in the body ; 
for a large and constant supply of it is required, to unite with 
‘the others and carry them off in their altered conditions. 
us, unless oxygen were continually introduced into the 
system, for the sake of uniting with the carbon that is to be 
thrown off by Respiration, that excretion must be checked ; 
“and it is required, in like manner, for uniting with hydrogen 
) 
= * 
i i OU 
