14 INTRODUCTION. 
nerves that produce them are even differently arranged from those that 
are appropriated to sensation, or dependent on the will, and the very object 
of this difference appears to be the securing of this independence. 
The nervous functions, that is, sensibility and muscular irritability, are 
so much the stronger at every point, in proportion as their exciting cause 
is abundant; and as this cause, or the nervous fluid, is produced by secre- 
tion, 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 circulating system, the blood is propelled through 
the arteries which convey it to its destined parts, by means of their irrita- 
bility and that of the heart. If these arteries be irritated, they act more 
strongly, and propel a greater quantity of blood; the nervous fluid be- 
comes more abundant and augments the local sensibility; this, in its turn, 
augments the irritability of the arteries, so that this mutual action may 
sometimes be carried to a great extent. It is called orgasm, and when 
it becomes painful and permanent, énflammation. The irritation may also 
originate in the nerve when exposed to the influence of acute sensations. 
This mutual influence of the nerves and fibres, either intestinal or ar- 
terial, is the real spring of vegetative life in animals. 
As each external sense is permeable only by such or such sensible sub- 
stances, so each internal organ may be accessible only to this or that agent 
of irritation. Thus, mercury irritates the salivary glands—cantharides 
irritate the bladder, &c. These agents are called specifics. 
The nervous system being homogeneous and continuous, local sensations 
and irritation debilitate the whole, and each function, by excessive action, 
may weaken the others. Excess of aliment weakens the power of thought, 
while long continued meditation impairs that of digestion, &c. 
Excessive local irritation will enfeeble the whole body, as if all the 
powers of life were concentrated in one single point. 
A second irritation produced at another part may diminish, or divert, as 
it is termed, the first: such is the effect of blisters, purgatives, &c. 
Brief as our sketch has been, it is sufficient to establish the possibility 
of accounting for all the phenomena of physical life, from the properties it 
presents, by the simple admission of a fluid such as we have defined. 
Summary Idea of the Functions and Organs of the Bodies of Animals, 
and of their various Degrees of Complication. 
After what we have stated respecting the organic elements of the body, 
its chemical principles and acting powers, nothing remains but to give a 
summary idea of the functions of which life is composed, and of their ap- 
propriate organs. 
