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. By Haier, 
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 m the ratio of the number of its 
muscular fibres. Different muscles, mm 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- 
ptiety 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 
