INTRODUCTION. 
23 
The will causes the fibre to contract through the medium of the nerve ; and the 
involuntary fibres, such as those we have mentioned, are equally animated by the 
nerves which pervade them ; it is, therefore, probable, that these nerves are the cause 
of their contraction. 
All contraction, and, generally speaking, all change of dimension in nature, is produced 
by a change of chemical composition, though it consists merely in the flowing or ebbing 
of an imponderable *, such as caloric ; it is thus also that the most violent of known 
movements are occasioned, as combustions, detonations, &c. 
There is, then, great reason for supposing that it is by an imponderable fluid that 
the nerve acts upon the fibre ; and the more especially, as it is demonstrated that this 
action is not mechanical. 
The medullary matter of the whole nervous system is homogeneous, and m.ust 
exercise, wherever it is found, the functions appertaining to its nature ; all its ramifi- 
cations receive a great abundance of blood-vessels. 
All the animal fluids being derived from the blood by secretion, it cannot be doubted 
that the same holds with the nervous fluid, nor that the medullary matter secretes 
[or evolves] it. 
On the other hand, it is certain that the medullary matter is the sole conductor 
of the nervous fluid ; and that all the other organic elements serve as non-conductors, 
and arrest it, as glass arrests electricity. 
The external causes which are capable of producing sensations, or of occasioning 
contractions in the fibre, are all chemical agents, capable of effecting decompositions, 
such as light, caloric, the salts, odorous vapours, percussion, compression, &c. 
It would seem, then, that these causes act upon the nervous fluid chemically, and 
by changing its composition : which appears the more likely, as their action becomes 
weakened by continuance, as if the nervous fluid needed to resume its primitive com- 
position in order to be altered anew. 
The external organs of sense may be compared to sieves, which allow nothing to 
pass through to the nerve except the species of agent which should affect it in that 
particular place, but which often accumulates so as to increase the effect. The 
tongue has its spongy papillae, which imbibe saline solutions : the ear a gelatinous 
pulp, which is intensely agitated by sonorous vibrations ; the eye transparent lenses, 
which concentrate the rays of light, &c. 
It is probable that what are styled irritants, or the agents which occasion the con- 
tractions of the fibre, exert this action by producing on the fibre, by the nerve, the 
same effect which is produced by the will ; that is to say, by altering the nervous fluid 
in the manner necessary to change the dimensions of the fibre on which it has influence ; 
but the will has nothing to do in this action ; the me is often even without any 
knowledge of it. The muscles separated from the body are still susceptible of irrita- 
tion, so long as the portion of the nerve distributed within them preserves its power of 
acting on them ; the will being evidently unconnected with this phenomenon. 
The nervous fluid is altered by muscular irritation, as well as by sensation and 
voluntary motion ; and the same necessity occurs for the re-establishment of its primi- 
tive composition. 
The movements of translation necessary to vegetative life are determined by irritants : 
* “ Imponderable fluid’' is the expression in the original. — E d. 
