m1 INTRODUCTION. 
between the highest of the lower animals and Man, of all beings, alone 
endowed with the power of reason and the faculty of speech, a dis- 
tance still more incalculable. 
Animals have been defined to be organized bodies, which have life 
and sensation, and are capable of voluntary motion ;—Vegetables. 
organized bodies, endowed. with a vital principle, but wanting sensa 
tion ;—and Minerals, unorganized bodies, without life, and, of course, 
without sensation. 
It has been found impossible to give a satisfactory definition of Lefe ; 
and physiological writers have therefore limited their efforts to com- 
municate some idea of the vital principle, by remarking its effects. 
Life, where its effects are most easily recognized, seems to consist in 
the faculty with which certain corporeal combinations are endowed, 
of existing for a certain period under a determinate form, and assimi- 
lating to their substance a part of the surrounding bodies; at the same 
time restoring to the elements part of their own substance. This 
vital principle, which, when allied to matter, controls its affinities and 
directs its forms, is not palpable to the senses in an uncombined shape ; 
and it is only from its effects on material substances, that its existence 
is demonstrated. Baron Cuvier compares the mechanical action of 
life on matter to a vortex, more or less rapid, more or less complicated, 
where the supply and the waste of particles occasion a constant move- 
ment. While this movement subsists, the body which exercises it 
lives; when the movement is stopped beyond recall, the body dies. 
After death, the elements which composed it, delivered to the ordi- 
nary chemical affinities, soon separate, to form other and new com- 
binations. 
All living bodies die, after a period, of which the limit is determi- 
nate for each species; and death, indeed, appears to be a necessary 
result of vital action, which insensibly alters the organic structure. 
The living body, which derives its mysterious birth from another 
living body which has preceded it, at first enlarges in dimensions, 
according to certain proportions and limits fixed for each species, and 
for each of its parts; these parts gradually increase in density; the 
fibres and vessels which compose them, imperceptibly acquire a rigid- 
ity, which: unfits them for the discharge of their functions ; the vital 
impulse ceases, and the body naturally dies. In short, absorption, 
assimilation, exhalation, developement, and generation, are functions 
common to all living beings; their birth and their death, the universal 
terms of their existence. 
