12 INTRODUCTION. 



blood, and it is oiily 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 chaiige 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- 

 loric; 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 

 so, as it is proved that this action is not mechanical. 



