10 INTRODUCTION. 
oxygen and azote; and carbonic acid, which 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 which 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 ihe 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 medullary matter; and to 
each form belongs a peculiar combination of chemical elements, as well 
as a particular function. 
The cellular substance is composed of an infinity of small fibres and 
lamine, 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 lamin called membranes ; 
the membranes rolled inte cylinders, form those more or less ramified tubes 
