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 [7:12:457]a sensible part of the human body, which, upon being 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 experiments, the periosteum and pericranium, the dura and pia mater, appeared insensible; and he infers, that the sensibility of the nerves is owing to the medulla, and not to the membranes. The arteries and veins are held to be susceptible of little or no sensation, excepting the carotid, the lingual, temporal, pharyngal, 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 sensation. 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 proportion 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 formation 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 sensible 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 tendons 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 genitals, are all irritable; as are also the muscles, particularly the diaphragm. The oesophagus, stomach, and intestines, are irritable; but of all the animal organs, the heart is endued 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, altogether independently of the influence of the soul. The irritability 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 different from such as serve either for voluntary motion or sensation. This doctrine, however, has been controverted by Μ. le Cat, and particularly by Dr Whytt, in his Physiological Essays.