84 ZOOLOGY. 



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 

 liv 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 

 membrane, 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 for their growth and support, each choosing the mole- 

 cules identical with its own nature. 



It is this last act which constitutes auimilatufi. 



169. But nothing is known as to the real nature of this 

 act of assimilation, how brought about, how ellirted. Such ques- 

 tions touch too nearly the very essence of the principle of life, 

 itself perfectly unknown in its nature. One thing is certain, 

 that iii all animals j o-s.-xin^ a nervous system, the influence 

 which this exercises over assimilation is distinct and unde- 

 niable. Nor is the duration of life in the various organs of 





