18 INTRODUCTION. 
the very object of this difference appears to be the securing 
of this independence. 
The nervous functions, that is, sensibility and muscular ir- 
ritability, are so much the stronger at every point, in pro- 
portion as their exciting cause is abundant; and as this cause 
or the nervous fluid is produced by secretion, its abundance 
must be in proportion to the quantity of medullary or secre- 
tory matter, and the amount of blood received by the latter. 
In animals that have a circulating system, the blood is pro- 
pelled 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 strongly, and pro- 
pel a greater quantity of blood; the nervous fluid becomes 
more abundant and augments the local sensibility; this, in its 
turn, augments the irritability of the arteries, so that this mu- 
tual action may sometimes be carried to a great extent. It is 
called orgasm, and when it becomes painful and permanent, 
imflammation. 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 in- 
testinal or arterial, is the real spring of vegetative life in ani- 
mals. 
As each external sense is permeable only by such or such 
sensible substances, so each internal organ may be accessible 
only to this or that agent of irritation. Thus, mercury irri- 
tates the salivary glands, cantharides irritate the bladder, &e. 
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. Ex- 
cess of aliment weakens the power of thought, while long con- 
tinued 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 blis- 
ters, purgatives, &e. ; 
Brief as our sketch has been, it is suflicient to establish the 
