88 ZOOLOGY. 



The primary materials of the organism are carbon, nitrogen 

 or azote, hydrogen and oxygen ; but sulphur, phosphorus, 

 lime, and other simple bodies, are also required; it is essen- 

 tial, then, that such bodies should be introduced from with- 

 out. But animals do not possess the faculty of determining 

 the combination of these various chemical elements so as to 

 give rise to the various compound principles of which the 

 organism is formed ; or, in other words, these elements must 

 be already combined. Thus, it is not by introducing azote, 

 hydrogen, carbon, &c., into the body that an animal can 

 satisfy the wants of nutrition ; these substances must have 

 been already combined. 



In a word, these principles must have been already com- 

 bined, so as to form organizable principles or viable matters. 

 Now, this only happens through the influence of life. It is 

 the vegetable kingdom, then, which directly or indirectly 

 furnishes to animal bodies the carbon and azote, and a certain 

 portion of hydrogen and oxygen ; water furnishes the greater 

 portion of the requisite hydrogen and oxygen ; the lime and 

 various other mineral bodies come directly from the mineral 

 kingdom. 



From the atmosphere, animals derive the oxygen required 

 to consume the carbon and hydrogen ; and thus, in brief, to 

 meet the wants of the nutritive process, every animal requires 

 to convey into the interior of its organization, free oxygen, 

 organized matters rich in carbon, hydrogen, azote, water, and 

 various salts. 



Before being adapted for nutrition, all substances must 

 assume a liquid or gaseous form; this is the object of diges- 

 tion. There exist three modes of ingress for the nutritive 

 matter the skin, the respiratory mucous membrane, the 

 alimentary canal. 



In man and animals which have an epidermis, absorption 

 by the skin is comparatively unimportant,-- by the lungs, some 

 liquid in the form of vapour is no doubt absorbed; but the 

 intestinal or alimentary canal, by means of its mucous mem- 

 brane, is the great route by which the matter destined to 

 assist in nutrition reaches the interior of the body. 



168. These nutritive elements are at first mingled with 

 the blood. This fluid, elaborated by processes not yet dis- 

 covered, becomes rich in all the compound principles of which 

 the tissues are in their turn formed; and it is out of this 

 fluid that all the organs of the body draw the materials fitted 



