OF EXCRETION AND SECRETION. 293 



ae purposes to which it is destined, it is requisite that these 

 >roducts should be drawn-off from the current of the circula- 

 don, as constantly as they are received into it ; and this is 

 accomplished by the various processes 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 Eespiration, 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, ix.) The effects of the suspension of 

 v he respiratory process, even for a few minutes, in a warm- 

 )looded 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. 

 Thus, 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 



