10 INTRODUCTION. 



oxygen and azote; and carbonic acid, whicli is a combination of oxygen 

 and carbon. To extract their own composition from these aliments, it was 

 necessary they should retain the hydrogen and carbon, exhale the super- 

 fluous oxygen, and absorb little or no azote. Such, in fact, is vegetable 

 life, whose essential function is the exhalation of oxygen, which is effected 

 through the agency of light. 



Animals also derive nourishment, directly or indirectly, from the vege- 

 table itself, in which hydrogen and carbon form the principal parts. To 

 assimilate them to their own composition, they must get rid of the su- 

 perabundant hydrogen and carbon in particular, and accumulate more 

 azote, which is performed through the medium of respiration, by which 

 the oxygen of the atmosphere combines with the hydrogen and carbon ot 

 their blood, and is exhaled with them in the form of water and carbonic 

 acid. The azote, whatever part of the body it may penetrate, seems al- 

 ways to remain there. 



The relations of vegetables and animals to the surrounding atmosphere 

 are therefore in an inverse ratio — the former reject water and carbonic acid, 

 while the latter produce them. The essential function of the animal 

 body is respiration, it is that whicli in a manner animalizes it, and we 

 shall see that the animal functions are the more completely exercised in 

 proportion to the greatness of the powers of respiration possessed by the 

 animal. This difference of relations constitutes the fourth character of 

 animals. 



Of the Forms peculiar to the Organic Elements of the Animal Body, and 

 of the principal Combinations of its Chemical Elements. 



An areolar tissue and three chemical elements are essential to every 

 living body ; there is a fourth element peculiarly requisite to that of an 

 animal ; but this tissue is composed of variously formed meshes, and these 

 elements are variously combined. 



There are three kinds of organic materials or forms of texture — the cel- 

 lular membrane, the muscular fibre, and the medtdlary matter; and to 

 each form belongs a peculiar combination of chemical elements, as well 

 as a particular function. 



The cellular sulstance is composed of an infinity of small fibres and 

 laminae, fortuitously disposed, so as to form little cells that communicate 

 with each other. It is a kind of sponge, which has the same form as the 

 body, all other parts of which traverse or fill it, and contracting indefi- 

 nitely, on the removal of the causes of its tension. It is this power that 

 retains the body in a given form and with certain limits. 



When condensed, this substance forms those laminae called membranes; 

 the membranes rolled into cvlinders, form those more or less ramified tubes 



