INTRODUCTION. 17 
agents, capable of effecting decompositions, such as light, 
caloric, the salts, odorous vapours, percussion, compression, 
&e. &e. 
It would appear then that these causes act on the nervous 
fluid chemically, and by changing its composition; this ap- 
pears the more likely, as their action becomes weakened by 
continuance, as if the nervous fluid needed the resumption of 
its primitive composition, to fit it for a fresh alteration. 
The external organs of the senses may be compared to 
sieves, which allow nothing to pass through to the nerve, ex- 
cept that species of agent which should affect it in that par- 
ticular place, but which often accumulates it so as to increase 
its effect. The tongue has its spongy papille which imbibe 
saline solutions; the ear, a gelatinous pulp which is violently 
agitated by sonorous vibrations; the eye, transparent lenses 
which concentrate the rays of light, &c. &c. 
It is probable, that what are styled irritants, or the agents 
which occasion the contractions of the fibre, exert this ac- 
tion by producing on the fibre, by the nerve, a similar effect 
to that produced on it by the will; that is, by altering the ner- 
vous fluid, in the way that is requisite to change the dimen- 
sions of the fibre which it influences: but with this process 
the will has nothing to do, and very often the ME is entirely 
ignorant of it. The muscles separated from the body pre- 
serve their susceptibility of irritation, as long as the portion 
of the nerve that remains with them preserves the power of 
acting on them—with this phenomenon the will has evidently 
no connexion. : 
The nervous fluid is altered by muscular irritation, as well 
as by sensibility and voluntary motion, and the same necessity 
exists for the re-establishment of its primitive composition. 
The transmutations necessary to vegetable life are occasioned 
by irritants; the aliment irritates the intestine, the blood irri- 
tates the heart, &e. These movements are all independent 
of the will, and generally (while in health) take place without 
the knowledge of the me; in several parts, the nerves that 
produce them are, even differently arranged from those that 
are appropriated to sensation or dependent on the will, and 
Vor. I.—C 8 
