INTRODUCTION. 21 



water and carbonic acid. The azote, whatever part of t'leir body it may penetrate, 

 appears to remain there. 



The relations of vegetables and animals with the atmosphere are then inverse ; the 

 former retain {defont) water and [decompose] carbonic acid, while the latter reproduce 

 them. Respiration is the function essential to the constitution of an animal body ; it 

 is that which in a manner animalizes it ; and we shall see that animals exercise their 

 peculiar functions more completely, according as they enjoy greater powers of respira- 

 tion. It is in this difference of relations that the fourth character of animals consists. 



OF THE FORMS PECULIAR TO THE ORGANIC ELEMENTS OF THE ANIMAL BODY, ANU OK 

 THE PRINCIPAL COMBINATIONS OF ITS CHEMICAL ELEMENTS. 



An areolar tissue and three chemical elements are essential to every living body, a 

 fourth element being peculiar to that of animals ; but this tissue is composed of vari- 

 ously formed meshes, and these elements are united in different combinations. 



There are three kinds of organic materials, or forms of tissue, — the cellular membrane, 

 the muscular fibre, and the medullary matter; and to each form belongs a peculiar 

 combination of chemical elements, together v.'ith a particular function. 



The cellular membrane is composed of an infinity of small laminae, fortuitously dis- 

 posed, so as to form little ceils that communicate with each other. It is a sort of 

 sponge, which has the same form as the entire body, all other parts of which fill or 

 traverse it. Its property is to contract indefinitely when the causes which sustain 

 its extension cease to operate. It is this force that retains the body in a given form, 

 and within determined limits. 



When condensed, this substance forms those more or less extended laminae which 

 are called membranes ; the membranes, rolled into cylinders, compose those tubes, more 

 or less ramified, which are termed vessels ; the filaments, named fibres, resolve them- 

 selves into it ; and the bones are nothing but the same, indurated by the accumulation 

 of earthy particles. 



The cellular substance consists of that combination [isinglass] which bears the 

 name of gelatine, and the character of which is to dissolve in boiling water, and to 

 assume the form, when cold, of a trembling jelly. 



The medullary matter has not yet been reduced to its organic molecules : it ap- 

 pears to the naked eye as a sort of soft bouilUe [pultaceous mass] , consisting of exces- 

 sively small globules ; it is not susceptible of any apparent motion, but in it resides 

 the admirable power of transmitting to the me the impressions of the external senses, 

 and of conveying to the muscles the mandates of the will. The brain and the spinal 

 chord are chiefly composed of it ; and the nerves, which are distributed to all the 

 sentient organs, are, essentially, but ramifications of the same. 



The fleshy or muscular fibre is a peculiar sort of filament, the distinctive property 

 of which, during life, is that of contracting when touched or struck, or when it experi- 

 ences, through the medium of the nerves, the action of the will. 



The muscles, immediate organs of voluntary motion, are merely bundles of fleshy 

 fibres. All the membranes, all the vessels which need to exercise any compression, are 

 furnished with these fibres. They are always intimately connected with nervous 

 threads ; but those which sul)serve the purely vegetative functions contract without 



