INTRODUCTION. oO 
The medullary matter of the whole nervous system is homogeneous, and 
must be able to exercise its peculiar functions wherever it is found; all its 
ramifications are abundantly supplied with blood vessels. 
All the animal fluids being drawn from the blood by secretion, we can 
have no doubt that such is the case with the nervous 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 organic elements restrain and 
arrest it, as glass arrests electricity. 
The external causes which are capable of producing sensations or caus 
ing contractions of the fibre are all chemical agents, capable of effecting 
decompositions, such as light, caloric, the salts, odorous vapours, percus- 
sion, compression, &c. &c. 
It would appear then that these causes act on the nervous fluid chemi- 
cally, and by changing its composition; this appears 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, except that species of agent 
which should affect it in that particular place, but which often accumulates 
it so as to increase its effect. The tongue has its spongy papilla which 
imbibe saline solutions; the ear, a gelatinous pulp which is violently agi- 
tated by sonorous vibrations; the eye, transparent lenses which concen- 
trate the rays of light, &c. &c. 
It is probable, that what are styled irritants, or the agents which occa- 
sion the contractions of the fibre, exert this action by producing on the 
fibre, by the nerve, a similar effect to that produced on it by the will; that 
is, by altering the nervous fluid, in the way that is requisite to change the 
dimensions of the fibre which it influences: but with this process the will 
has nothing to do, and very often the mr is entirely ignorant of it. The 
muscles separated from the body preserve 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 sen- 
sibility 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 irri- 
tants; the aliment irritates the intestine, the blood irritates the heart, &c. 
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 
