(From irrito, to provoke). Irritability; the vis insita of Haller, vis vitalis of Gorter, oscillation of Boerhaave, tonic power of Stahl, and the inherent power of Cullen. It means that susceptibility to contraction which is peculiar to muscular fibres. We chiefly speak of it when morbidly increased, and in this state it may exist without or with inflammation. In the former state it is called spasm; in the latter, it is considered as a symptom only. Parts scarcely sensible or irritable in a natural state, become highly so in consequence of inflammation.

Irritability, according to Haller, differs greatly from sensibility; for many irritable parts are not sensible; and organs which are both irritable and sensible have by no means these qualities in the same or any proportional degree. The intestines, he remarks, are less sensible than the stomach, though more irritable; and the heart is an organ peculiarly irritable, though by no means sensible. In the language of this physiologist, cellular is often considered as synonymous with nervous, and, both in the French and German authors, the cellular substance is spoken of as an important organ, often as an irritable one. We have already offered our opinion, that it is merely an insensible connecting medium; nor have we been ever able to ascertain that it has any other office, or any appropriate function. The cellular, or rather the membranous parts, he considers as irritable, particularly the ligaments, the periosteum, the dura and pia mater, and the other membranes. The tendons possess, he thinks, no irritability; and, though the smaller arteries may possess this quality, he did not discover it in the aorta. The veins, the excretory ducts, the gall bladder and its ducts, the urethra, and ureters, he found only irritable in an inconsiderable degree; but the glands, the mucous sinuses, the uterus, the genitals, the oesophagus, the stomach, the intestines, the muscles, and particularly the diaphragm, are highly irritable. This principle he supposes to be owing to the mucous matter interspersed among the muscular fibres, and to be wholly independent of volition. Other authors have attributed irritability to a particular set of nerves not under the influence of the mind. It is certain that it may be destroyed by drying; lating the oily fluids of our system; by opium, belladonna, tobacco, etc.

The high reputation of Haller has induced us to enlarge on his opinions much farther than their real merit would require. His experiments were made on animals, often cold blooded ones, and in a state of health. He seems never to have looked at the human body in a state of disease, as the source of his observations; but had he done so, he would have found numerous facts in the most decided contradiction to his experiments. No part, not the eye itself, is more sensible than, for in stance, the membranes when inflamed; no part show-greater irritability, either from passions or the stimulus of a gall stone, than the biliary ducts. The idea, that irritability is owing to the mucus in the interstices of the muscular fibres, is gratuitous and imaginary. Irritability is a property of life; but by what means does this mucus acquire life, and by what function, except this fancied one, does it show any vital power? The existence of different sets of nerves giving irritability is wholly imaginary, without the slightest support from anatomy.

Irritability, as inseparable from life, must be col nected with the nervous power; but the nerves, We know, are not irritable. It must then owe its existence to the nervous influence, or the muscular fibre must differ from the nervous by some peculiar organization on which this property depends. There is little doubt of the muscular power depending on organization; for the muscle differs only from the tendon in structure. The fibres pass on, and may be traced from one to the other. Organs, at first in a great degree muscular,become, by age, more tendinous; so that, in the latter, the fibres are apparently more compacted, in fact, of an organization essentially different. Is then the muscle wholly nervous, or an organ only excited to action by nerves? Dr. Cullen thinks it wholly nervous, and calls muscles the moving extremities, in opposition to the sentient extremities of nerves; and the weight of evidence is strongly in favour of this opinion. We have said that we know of animal matter but in two forms. fibrous and cellular substance, more or less condensed. The fibrous seems exclusively nervous, and the nerves are closely compacted as in membranes, or more loosely organized in muscles. It is equally difficult to understand the construction of muscles, whether we supposed them nervous, or animal matter of any other kind.

Though we cannot ascertain the structure of the organs possessing irritability, we may shortly mention the laws by which it is regulated; and we shall find them so analogous to those of sensibility, that there will be little difficulty in recognizing the source to be similar. Irritability, like sensibility, is exhausted by exercise, and recruited by sleep; but, unlike sensibility, its exertions are alternated by relaxation. It is probable that the nerves in the muscular organs are in a higher state of excitement than in the sentient; for muscular organs are constantly, in a certain degree, exerted in order to counteract the antagonizing muscles, but unless the action is occasionally remitted, it is spontaneously relaxed. This constant state of tension is called the tonic flower, and is in proportion to the general excitement. It presupposes irritability, in consequence of organization, which has been called the inherent power, and is probably the same with the ner-vous power of physiologists: it must be so if muscles are only the "moving extremities of nerves." Irritability, as a morbid affection, however, implies a very different state, and has been styled mobility, a susceptibility of action from slight and otherwise insufficient stimuli; and this susceptibility, connected generally with debility, is more readily alternated with relaxation producing convulsions.

Irritability, as we have just remarked, is exhausted by exercise, and it may be suddenly destroyed, so as to kill; for a flash of lightning will at once extinguish it in every organ. It is, however, recruited by rest, and, as modern physiologists, with some reason, have supposed, accumulated so as to occasion increased action. Many of the facts adduced may, however, be resolved into the effects of custom; for, when given actions are excited by a weak stimulus, a common power becomes, in comparison, inordinate. Thus the iris, accustomed to contract in the gloom of a dungeon, feels the common daylight as painful as the glare of a noon day sun would be to a person who has never been confined. This leads us to remark, that the irritability of each organ has specific stimulus, by which only it is excited. Ipecacuanha does not irritate the eye; and the acrid urine or bile excite only pleasing and healthy sensations in their appropriate organs; but in the brain produce phrenitis, or in the stomach, vomiting, with faintness, cold sweats, etc. In general, muscles which act more slowly and regularly, preserve their irritability longer than those which act with violence; for in these the irritability is apparently supplied as fast as it is expended. A certain degree of action, we have remarked, must be kept up in all muscles, to preserve their irritability, or rather the tonic power. Beyond, irritability is exhausted; below the due point, it is lost; and this point differs in almost all the different organs. Each has its appropriate action, which it can bear without injury, or even with advantage; and the irritability of each is exhausted more rapidly, in proportion to the continued action which it exerts. The voluntary muscles can bear a very considerable increase of action, because it is temporary: the involuntary ones, whose action must continue, soon lose their irritability after a short increase. See Nervi; Musculi; Irritatio, and Cerebrum.

Haller on Sensibility and Irritability; Whytt's Answer to Haller; the Difficulties in the Modern System of Physic, with Regard to the Sensibility and Irritability of the Parts of the Human Body, by De Haen; Kirkland on the Brain and Nerves; on the Sympathy of the Nerves, and of different Kinds of Irritability; Cullen's Introduction to the Materia Medica.