CHAP, ix.] OF THE MEANS OF ANIMAL NUTRITION. 155 



given back to the elements in the degree of their formation. A 

 special apparatus performs the task of conveying to each 

 anatomical element, whatever may be the situation and function 

 of that element in the organism, new materials, while at the 

 same time it carries away, sweeps off the nutritive residua. 

 This apparatus is the circulatory system. 



Moreover, an incessant atmospheric ventilation is an indis- 

 pensable condition of the ultimate nutritive exchange. Every 

 living substance needs to combine with the oxygen of the air, 

 and to exhale carbonic acid, which is one of the principal products 

 of this combination. The special apparatus which provides an 

 easy entrance and issue to the circulating gases in the complex 

 organism has been called the respiratory system. 



But the respiratory apparatus can only provide an issue for 

 gaseous products ; now denutrition, as we have seen, originates a 

 whole family, yea, even several families, of regressive products, 

 salts, quaternary substances. These bodies are taken afresh from 

 the anatomical elements by the circulatory system; but this 

 system would be incapable, by itself, at least in a very complex 

 organism, of removing them definitively beyond the frontiers of 

 the living histological federation, which constitutes every superior 

 organism. For this there must be special apparatus, excretory 

 glands. * 



For the sake of clearness of exposition, we are obliged thus 

 to cut the complex organism into a certain number of great 

 divisions ; but in reality, in the living being, all these functions 

 are bound up together, are indispensable to each other, and are 

 carried on simultaneously, to such a point, indeed, that, to under- 

 stand the working of one of them, it is almost necessary to know 

 the working of them all. Consequently, in giving an exposition 

 of them successively in the order of our enumeration, we shall 

 be compelled to leave behind us, on the way, many gaps, many 

 obscure points, which will only disappear in the course of the 

 exposition. Finally, when we shall have passed in review 

 digestion, circulation, respiration, and excretion, it will remain 



