IRRITABILITY

IRRITABILITY, in Anatomy and Medicine, a term first invented by Glisson, and adopted by Haller, to denote an essential property of all animal bodies, which exists independently of, and in contradistinction to, sensibility. This

ingenious author calls that part of the human body irritable, which becomes shorter upon being touched; very irritable, if it contracts upon a slight touch; and the contrary, if by a violent touch it contracts but little. He calls that

rit- a sensible part of the human body, which, upon being
lity. touched, transmits the impression thereof to the soul; and
in brutes, he calls those parts sensible, the irritation of
which occasions evident signs of pain and disquiet in the
animal. On the contrary, he calls that insensible, which
being burned, torn, pricked, or cut till it is quite destroyed,
occasions no sign of pain or convulsion, nor any sort of
change in the situation of the body. From the result of
many cruel experiments, Haller concludes that the epidermis
is insensible; that the skin is sensible in a greater degree
than any other part of the body; that the fat and cellular
membrane are insensible; and that the muscular flesh is
sensible, its sensibility being ascribed by him rather to the
nerves than to the flesh itself. The tendons, he says, having
no nerves distributed to them, are insensible. The ligaments
and capsulae of the articulations are also believed to be
insensible; and hence Haller infers, that the sharp pains
of the gout are not seated in the capsulae of the joint, but
in the skin, and in the nerves which creep upon its external
surface. The bones are all insensible, says Haller, except
the teeth; and likewise the marrow. Under his experi-
ments, the periosteum and pericranium, the dura and pia
mater, appeared insensible; and he infers, that the sensi-
bility of the nerves is owing to the medulla, and not to the
membranes. The arteries and veins are held to be sus-
ceptible of little or no sensation, excepting the carotid, the
lingual, temporal, pharyngeal, labial, thyroidal, and the aorta
near the heart, the sensibility of which is ascribed to the
nerves that accompany them. Sensibility is allowed to the
internal membranes of the stomach, intestines, bladder,
ureters, vagina, and womb, on account of their being of the
same nature with the skin; the heart is also admitted to
be sensible, but the lungs, liver, spleen, and kidneys, are
possessed of a very imperfect, if any, sensation. The glands,
having few nerves, are endowed with only an obtuse sen-
sation. Some sensibility is allowed to the tunica choroides
and the iris, though in a less degree than the retina; but
none to the cornea. Haller concludes, in general, that the
nerves alone are sensible of themselves; and that, in pro-
portion to the number of nerves apparently distributed to
particular parts, such parts possess a greater or less degree
of sensibility.

Irritability, he says, is so different from sensibility, that
the most irritable parts are not at all sensible, and vice
versa. He alleges facts to prove this position, and also to
demonstrate, that irritability does not depend upon the
nerves, which are not irritable, but upon the original for-
mation of the parts which are susceptible of it. Irritability,
he says, is not proportioned to sensibility; and in proof of
this, he observes, that the intestines, though rather less sen-
sible than the stomach, are more irritable, and that the heart
is very irritable, though it has but a small degree of sensation.

Irritability, according to Haller, is the distinguishing
characteristic between the muscular and cellular fibres;
and hence he determines that the ligaments, periosteum,
meninges of the brain, and all the membranes composed of
the cellular substance, are void of irritability. The ten-
dons are not irritable; and though he does not absolutely
deny irritability to the arteries, yet his experiments on the
aorta produced no contraction. The veins and excretory
ducts are in a small degree irritable, and the gall-bladder,
the ductus choledochus, the ureters and urethra, are only
affected by a very acrid corrosive; but the lacteal vessels
are considerably irritable. The glands and mucous sinuses,
the uterus in quadrupeds, the human matrix, and the geni-
tals, are all irritable; as are also the muscles, particularly
the diaphragm. The œsophagus, stomach, and intestines,
are irritable; but of all the animal organs, the heart is en-
dued with the greatest irritability. In general, there is
nothing irritable in the animal body but the muscular
fibres; and the vital parts are the most irritable. This

power of motion, arising from irritations, is supposed to be
different from all other properties of bodies, and probably
resides in the glutinous mucus of the muscular fibres, alto-
gether independently of the influence of the soul. The irri-
tability of the muscles is said to be destroyed by drying the
fibres, congealing the fat, and more especially by the use
of opium in living animals. The physiological system, of
which an abstract has now been given, has been adopted
and confirmed by Castell and Zimmermann, and also by
Brocklesby, who suggests, that irritability, as distinguished
from sensibility, may depend upon a series of nerves dif-
ferent from such as serve either for voluntary motion or
sensation. This doctrine, however, has been controverted
by M. le Cat, and particularly by Dr Whytt, in his Phy-
siological Essays.