22 INTRODUCTION. 



These perpetual changes of chemical composition form a part of 

 the vital vortex, not less essential than the visible movements and 

 those of translation. The object of the latter is, in fact, but to pro- 

 duce the former. 



Of the forces which act in the Animal Body. 



The muscular fibre is not only the 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 con- 

 tained 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 contraction. 



All contraction, and generally speaking, every change of dimen- 

 sion in nature, is produced by a change of chemical composition, 

 though it consist merely in the flowing or ebbing of an imponderable 

 fluid, such as caloric ; thus also are produced the most violent move- 

 ments 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. 



The medullary matter of the whole nervous system is homoge- 

 neous, and must be able to exercise its peculiar functions wherever 

 it is found ; all its ramifications are abundantly supplied with blood 

 vessels. 



All the animal fluids being drawn from the blood by secretion, 

 we can have no doubt that such is the case with the nervous fluid, 

 and that the medullary matter secretes it. 



On the other hand, it is certain that the medullary matter is the 

 sole conductor of the nervous fluid ; all the other organic elements 

 restrain and arrest it, as glass arrests electricity. 



The external causes which are capable of producing sensations 

 or causing contractions of the fibre are all chemical agents, capable 

 of eflecting decompositions, such as light, caloric, the salts, odorous 

 vapours, percussion, compression, d:.c. &c. 



