INTRODUCTION. 
24 
the aliment irritates [or excites] the intestine, the blood irritates the heart, &c. These 
movements are all independent of the will, and in general (while health endures) take 
place without the cognizance of the me ; the nerves which produce them have even, 
in several parts, a different distribution from that of the nerves affected by sensations 
or subject to the will, and the object of the difference appears to be the securing of 
this independence.* 
The nervous functions, that is to say, sensitiveness and muscular irritability, are so 
much the stronger at every point, in proportion as the exciting cause is more abundant ; 
and as this agent, or the nervous fluid, is produced by secretion [or evolution], its 
abundance must be in proportion to the quantity of medullary or secretory matter, 
and the amount of blood received by the latter. 
In animals that have a circulation, the blood is propelled through the arteries which 
convey it to its destined parts, by means of their irritability and that of the heart. If 
these arteries be irritated, they act more vigorously, and propel a greater quantity of 
blood ; the nervous fluid becomes more abundant, and augments the local sensibility ; 
this, in its turn, increases the irritability of the arteries, so that this mutual action may 
be carried to a great extent. It is termed orgasm, and when it becomes painful and 
permanent, inflammation. The irritation may also originate in the nerve, when it 
experiences acute sensations. 
This mutual influence of the nerves and fibres, either in the intestinal system, or in 
the arterial system, is the real spring of vegetative life in animals. 
As each external sense is permeable only by particular kinds of sensation, so 
each internal organ may be accessible only to such or such agent of irritation. Thus, 
mercury irritates the salivary glands, cantharides excite the bladder, &c. These 
agents are what are termed speciflcs. 
The nervous system being homogeneous and continuous, local sensations and'irrita- 
tion debilitate the whole ; and each function, carried too far, may enfeeble the others. 
Excess of aliment thus weakens the faculty of thought ; while prolonged meditation 
impairs the energy of digestion, &c. 
Excessive local irritation will enfeeble the whole body, as if all the powers of life 
were concentrated on a single point. 
A second irritation produced at another point may diminish, or divert as it is termed, 
the first; such is the effect of purgatives, blisters, &c. [denominated counter-irritation]. 
All rapid as the foregoing enunciation is, it is sufficient to establish the possibility of 
accounting for all the phenomena of physical life, by the simple admission of a fluid 
such as we have defined, from the properties which it manifests. f 
* In the above sentence, there are ®stinctly mentioned the three 
sorts of nerves, the separate functions of which have been con- 
clusively demonstrated by Sir Charles Bell : viz., nerves of volition, 
which transmit the mandates of the will ; of sensation, which convey 
to the sensorium the impressions of the senses ; and of sympathy, 
or involuntary movement, the reunion of the ramifications of which 
in a plexus of knots, or ganglions, is intimated in the text, those of 
the second class being distinguished by a swelling or ganglion near 
their base. — Ed. 
t The unceasing chemical changes consequent upon vitality must 
necessarily develope electricity ; and that the nervous fluid is no other 
than the electric, may be considered as proved by the identity of their 
phenomena. Indeed, it has long been known that the transmission 
of voltaic electricity along the nerves of a recently dead animal, 
suffices to produce the most violent muscular action ; but the regula- 
tion of that action, its exclusive direction to particular suites of 
muscles, requires the vital impulse. “If the brain,” remarks Sir 
John Herschel, “ (for which wonderfully constituted organ no other 
mode of action possessing the least probability has ever been devised) , 
be an electric pile, constantly in action, it may be conceived to dis- 
charge itself at regular intervals, when the tension of the electricity 
developed reaches a certain point, along the nerves which communi- 
cate w'ith the heart, and thus to excite the pulsations of that organ. 
This idea is forcibly suggested by a view of that elegant apparatus, 
the dry pile of Deluc, in which the successive accumulations of 
electricity are carried off by a suspended ball, which is kept, by the 
discharges, in a state of regular pulsation for any length of time. AVe 
have witnessed the action of such a pile, maintained in this way for 
whole years, in the study of the above-named eminent philosopher. 
The same idea of the cause of the pulsation of the heart appears to 
have occurred to Dr. Arnott, and is mentioned in his useful and ex- 
cellent work on Physics, to which, however, we are not indebted for 
the suggestion, it having occurred to us independently many years 
ago.” — Discourse on the Study of Natural Philosophy, p. 343. — Ed. 
