16 INTRODUCTION. 



Of the forces which act in the Animal Body. 



The muscular fibre is not only the organ of voluntary mo- 

 tion, 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 intes- 

 tines produce the peristaltic motion, which causes the alimen- 

 tary 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 men- 

 tioned, 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 

 dimension 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 pro- 

 duced the most violent movements known upon earth, explo- 

 sions, &c. 



There is, consequently, good reason to suppose that the 

 nerve acts upon the fibre through the medium of an impon- 

 derable fluid, and the more so, as it is proved that this action 

 is not mechanical. 



The medullary matter of the whole nervous system is ho- 

 mogeneous, and must be able to exercise its peculiar func- 

 tions wherever it is found; all its ramifications are abundantly 

 supplied with blood vessels. 



All the animal fluids being drawn from the blood by secre- 

 tion, we can have no doubt that such is the case with the ner- 

 vous 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 or- 

 ganic elements restrain and arrest it, as glass arrests electri- 

 city. 



The external causes which are capable of producing sensa- 

 tions or causing contractions of the fibre are all chemical 



