PHYSIOLOGY. 
The brain wbicli is the true seat of this re- 
lative sensibility (or, as it might well be 
called, perceptibility), being excited by 
these impressions, influences the moving 
powers of the muscles, and determines the 
exercise of their contractility. This pro- 
perty, subjected to tlie command of the 
will, is manifested by the sudden shorten- 
ing of the muscular organ, which swells, be- 
comes hard, and causes those parts of the 
skeleton to which it is attached to move. 
The nerves and the brain are the essential 
organs of these two properties ; division of 
the former destroys sensation, and the vo- 
luntary motion of those parts to which the 
nerves are distributed. But there is an- 
other kind of sensibility quite independent 
of the presence of nerves, existing in all or- 
gans, even where no nervous filaments are 
distributed. Bones, cartilages, ligaments, 
arteries, and veins, in short, all parts which 
are not influenced by the will, possess no 
nerves. Yet, though in their natural state 
they transmit to the brain no perceptible 
impression, though they may be injured 
v/ithout giving the animal any pain, and 
though the will has no influence over them, 
yet they enjoy a sensibility and contracti- 
lity, by virtue of which they perceive im- 
pressions, and contract in their own man- 
ner, recognize in the fluids which circulate 
through them what is proper for their nu- 
trition, and, separating this part, appro- 
priate it to their own substance. 
We recognize then in the parts of our 
body two modes of sensation, as well as 
two species of motion: a sensibility, by 
means of which certain parts transmit to 
the brain, impressions which they feel, and 
of which we therefore become conscious : 
a second kind, pervading every part with- 
out exception, and presiding over tlie assi- 
milating functions. We observe also two 
kinds of contractility corresponding to the 
diflferences of sensibility : the one by which 
the voluntary muscles perform the contrac- 
tions determined by the action of the will ; 
the other manifested by actions which are 
equally unknown as the causes which give 
rise to them. 
When we have once clearly distinguished 
these two grand m,odifications of sensibility 
and contractility, we shall find out, without 
difficulty, the source of those eternaf dis- 
putes, raised by Haller and his followers, 
concerning the irritable and sensible na- 
ture of parts. Bones, tendons, cartilages, 
&c, to which this great physiologist denied 
these two properties, enjoy only that late* 
ral sensibility and obscure contractility 
which are common to all living beings, and 
without which we cannot conceive the ex- 
istence of life. In the healthy state they 
are completely destitute of the power of 
transmitting perceptible impressions to the 
brain, or of being influenced by that organ 
to any manifest motion. It has also been 
disputed whether sensibility and contracti- 
lity depended on the existence of nerves ; 
whether these were its necessary instru- 
ments, and whether their injuries were fol- 
lowed by a loss of those vital powers in the 
parts which have nerves. We may answer 
in the affirmative, as far as regards percep- 
tive sensibility, and voluntary motion, which 
is entirely subordinate to it; but in the 
negative with respect to the sensibility and 
contractility which are indispensable in the 
processes of assimilation. 
Sensibility may then be either perceptive 
or latent. The former is attended with a 
conscience of the impressions or percepti- 
bility, and requires a peculiar apparatus. 
The latter, unaccompanied by conscious- 
ness, is common to every thing that lives ; 
it has no particular organs, but is univer- 
sally expanded in all living parts, whether 
of vegetables or animals. Contractility 
may be either voluntary and sensible (vis 
nervosa), which is subordinate to percep- 
tibility ; involuntary and insensible, which 
corresponds to latent sensibility ; or invo- 
luntary and sensible (vis itisita), as in the 
action of the heart, stomach, &c. 
The former species of sensibility being 
that which is observed in the functions, 
which connect the animal with external ob- 
jects, is called by Bichat, animal sensibi- 
lity; and the corresponding contractility is 
distinguished by the same term. The 
other kind of these two vital powers, which 
are exerted in the internal processes of nu- 
trition, &c. common to animals and plants, 
that is, to all organized bodies, is named 
the organic. 
Organic sensibility is merely the facnlty 
of receiving an impression ; animal sensi- 
bility is the same faculty, with the additi- 
onal power of conveying it to a common 
centre. In the former case the effect ter- 
minates in the organ. The latter belong* 
only to animals, whose perfection is in a 
direct ratio to the quantity of this sensibi- 
lity. There is some reason for supposing 
that these two are not different powers, 
bat, that they differ only in quantity. For 
