12 INTRODUCTION. 
blood, and it is only by possessing a few elements more or less, that each 
of them is distinguished; whence it is plain, that their formation entirely 
depends on the subtraction of the whole or part of one or more elements 
of the blood, and in some few cases, on the addition of some element from 
elsewhere. 
These operations, by which the blood nourishes the fluid or solid matter 
of all parts of the body, may assume the general name of secretions. This 
name, however, is often appropriated exclusively to the production of li- 
quids; while that of nutrition is more especially applied to the formation 
and deposition of the matter necessary to the growth and conservation of 
the solids. 
The composition of every solid organ, of every fluid, is precisely such as 
fits it for the part it is to play, and it preserves it as long as health re-~ 
mains, because the blood renews it as fast it becomes changed. The blood 
itself by this continued contribution is changed every moment, but is re- 
stored by digestion, which renews its matter by respiration, which delivers 
it from superfluous carbon and hydrogen, by perspiration and various other 
excretions, that relieve it from other superabundant principles. 
These perpetual changes of chemical composition form a part of the vi- 
tal vortex, not less essential than the visible movements and those of trans- 
lation. ‘The object of the latter is, in fact, but to produce the former. 
Of the Forces which act in the Animal Body. 
The muscular fibre is not the only organ of voluntary motion, for we 
have just seen that it is also the most powerful of the agents employed by 
nature to produce those transmutations so necessary to vegetative life. 
Thus the fibres of the intestines produce the peristaltic motion, which 
causes the alimentary matter therein contained to pass through them; the 
fibres of the heart and arteries are the agents of the circulation, and through 
it of all the secretions, &c. 
Volition contracts the fibre through the medium of the nerve; and the 
involuntary fibres, such as those we have mentioned, being also animated 
by them, it is probable that these nerves are the cause of their con- 
traction. 
All contraction, and, generally speaking, every change of dimension in 
nature, is produced by a change of chemical composition, though it con- 
sist merely in the flowing or ebbing of an imponderable fluid, such as ca- 
lorié; thus also are produced the most violent movements known upon 
earth, explosions, &c. 
There is, consequently, good reason to suppose that the nerve acts 
upon the fibre through the medium of an imponderable fluid, and the more 
Sc, as it is proved that this action is not mechanical. 
