6 INTRODUCTION. 
and from which they are continually departing; so that the form of a living 
body is more essential to it than its matter. 
As long as this motion subsists, the body in which it takes place is 
living—it lives. When it finally ceases, zt dies. After death, the ele- 
ments which compose it, abandoned to the ordinary chemical affinities, 
soon separate, from which, more or less quickly, results the dissolution of 
the once living body. It was then by the vital motion that its dissolution 
was arrested, and its elements were held in a temporary union. 
All living bodies die after a certain period, whose extreme limit is fixed 
for each species, and death appears to be a necessary consequence of life, 
' which, by its own action, insensibly alters the structure of the body, so as 
to render its continuance impossible. 
In fact, the living body undergoes gradual, but continual changes, dur- 
ing the whole term ofits existence. At first, it increases in dimensions, ac= 
cording to proportions, and within limits, fixed for each species and for each 
one of its parts; it then augments in density in the most of its parts—it is 
this second kind of change that appears to be the cause of natural death. 
If we examine the various living bodies more closely, we find they pos- 
sess a common structure, which a little reflection soon causes us to per- 
ceive is essential to a vortex, such as the vital motion. 
Solids, it is plain, are necessary to these bodies, for the maintenance of 
their forms; and fluids for the conservation of motion in them. ‘Their 
tissue, accordingly, is composed of network and plates, or of fibres and so- 
lid lamin, within whose interstices are contained the fluids; it is in these 
fluids that the motion is most continued and extended. Foreign substan- 
ces penetrate the body and unite with them; they nourish the solids by 
the interposition of their molecules, and also detach from them those that 
are superfluous. It is in a liquid or gaseous form that the matters to be 
exhaled traverse the pores of the living body; but in return, it is the so- 
lids which contain the fluids, and by their contraction communicate to them 
part of their motion. 
This mutual action of the fluids and solids, this transition of molecules, 
required considerable affinity in their chemical composition; and such is 
the fact—the solids of organized bodies being mostly composed of ele- 
ments easily convertible into fluids or gases. 
The motion of the fluids needing also a constantly repeated action on 
the parts of the solids, and communicating one to them, required in the 
latter both flexibility and dilatibility; and accordingly we find this charac- 
ter nearly general in all organized solids. 
This structure, common to all living bodies; this areolar tissue, whose 
more or less flexible fibres or laminz intercept fluids more or less abund- 
ant; constitutes what is called the organization. As a consequence of 
