INTRODUCTION. 19 



necessary that these should be most numerous in animals, and that 

 the chemical composition of the animal body be more complex than 

 that of the plant; and so it is, for one substance more (azote) enters 

 into it as an essential element, whilst in plants it is a mere acciden- 

 tal junction with the three other general elements of organization, 

 oxygen, hydrogen, and carbon. This then is the third character of 

 animals. 



From the sun and atmosphere, vegetables receive for their nutri- 

 tion water, which is composed of oxygen and hydrogen; air, which 

 contains oxygen and azote; and carbonic acid, which is a combina- 

 tion of oxygen and carbon. To extract their own composition from 

 these alinients, it was necessary they should retain the hydrogen 

 and carbon, exhale the superfluous 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 

 vegetable itself, in which the hydrogen and carbon form the principal 

 parts. To assimilate them to their own composition, they must get 

 rid of the superabundant 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 of 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 always to remain there. 



The relations of vegetables and animals to the surrounding atmos- 

 phere are therefore in an inverse ratio — the former reject water and 

 carbonic acid, while the latter produce them. The essential func- 

 tion 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 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 



