VITAL PRINCIPLE. 13 



suppose, every fleshy fibre is accompanied by a nervous 

 filament. Hence they conclude, that the peculiar motions 

 of a muscle, after the section of its nerve, depend upon the 

 portion of nervous pulp which it contains. Bv HAL-LEU, 

 and a few others, irritability is considered as a quality of 

 the muscular fibre itself; and they adduce in support of 

 their opinion, the well known facts, that parts not muscu- 

 lar are not irritable, and that no proportion exists between 

 the degree of irritability and the number of nerves in any 

 part. 



If the degree of irritability of any part be not in propor- 

 tion to the number or the size of the nerves with which it 

 is supplied, neither is it in the ratio of the number of its 

 muscular fibres. Different muscles, in the same animal, 

 possess dissimilar degrees of irritability; and the same 

 muscles, in different species, likewise vary in the intensity 

 of their action under similar exciting causes. It is impos- 

 sible to form a conception of irritability, in which the mus- 

 cular fibre shall not constitute the essential part. It is the 

 only portion of an organized body capable of exhibiting the 

 signs of its action ; and, by consequence, the only part in 

 which we have any ground to believe that it exists. The 

 nerves, where present, may excite to action the muscular 

 fibre, by acting as stimulants ; but the same actions of con- 

 traction and expansion are performed in animals in which 

 no nerves can be traced. In those animals, which belong 

 to the least perfect classes, the nervous pulp is supposed to 

 be diffused throughout the different parts, communicating 

 to the muscular fibre its susceptibility of irritation. There 

 is reason, indeed, from analogy, to acknowledge the pro- 

 priety of the conjecture respecting this dissemination of the 

 source of nervous energy in the cases to which we refer ; 

 otherwise, how could the will of those animals execute its 

 purposes in the production of spontaneous motion. But 



