IRRITABILITY

From Agepedia

IRRITABILITY (irritabilitas; fromirrito,to provoke;vis insita of Haller; vis vitalis of Gorter; oscillation of Boerhaave ; tonic power of Stahl; muscular power of Bell; inherent power of Cullen); the contractility of muscular fibres, or a property peculiar to muscles, by which they contract, upon the application of certain stimuli, without a consciousness of action. This power may be seen in the tremulous contraction of muscles when lacerated, or when entirely separated from the body in operations. Even when the body is dead, to all appearance, and the nervous power is gone, this contractile power remains till the organization yields, and hegins to be dissolved. It is by this inherent power that a cut muscle contracts, and leaves a gap, that a cut artery shrinks, and grows stiff after death. This irritability of raus<les is so far independent of nerves, and go little connected with feeling, which is the province of the nerves, that, upon stimulating any muscle by touching it with caustic, or irritating it with a sharp point, or driving the electric spark through it, or exciting with the metallic conductors, as those of silver or zinc, the muscle instantly contracts, although the nerve of that muscle be tied; although the nerve be cut so as to separate the muscle entirely from all connexion with the system; although the muscle be separated from the body; although the creature, upon which the experiment is performed, may have lost all sense of feeling and have been long apparently dead. Thus a muscle, cut from the limb, trembles and palpitates a long time after; the heart, separated from the body, contracts when irritated; the bowels, when torn from the body, continue their peristaltic motion, so as to roll upon the table, ceasing to answer to 'stimuli only when they become stiff and cold. Even in vegetables, as in the sensitive plant, this contractile power lives. Thence comes the distinction between the irritability of muscles and the sensibility of nerves; for the irritability of muscles survives the animals, as when it is active after death; survives the life of the part, or the feelings of the whole system, as in universal palsy, where the vital motions continue entire and perfect, and where the muscles, though not obedient to the will, are subject to irregular and violent actions; and it survives the connexion with the iest of the system, as when animals very tenacious of life, are cut into parts; but sensibility, the property of the nerves, gives the various modifications of sense, as vision, hearing, and the rest; gives also the general sense of pleasure or pain, and makes the system, according to its various conditions, feel vigorous and healthy, or weary and low. The eye feels and the skin feels; but their appointed stimuli produce no motions in these parts: they are sensible, but not irritable. The heart, the intestines, the urinary bladder, and all the muscles of voluntary motion, answer to stimuli with a quick and forcible contraction ; and yet they hardly feel the stimuli by which these contractions are produced, or, at least, they do not convey that feeling to the brain. There is no consciousness of present stimulus in those parts which are called into action by the impulse of the nerves, and at the command of the will; so that muscular parts have all the irritability of the system, with but little feeling, and that little owing to the nerves which enter into their substance ; while nerves have all the sensibility of the system, but no motion. After every action in an irritable part, a state of rest, or cessation from motion, must take place before the irritable part can be again incited to action. If, by an act of volition, we throw any of our muscles into action, that, action can only be continued for a certain space of time. The muscle becomes relaxed, notwithstanding all our endeavors to the contrary, and remains a certain time in that relaxed state, before it can be again thrown into action. Each irritable part has stimuli which are peculiar to it, and which are intended to support its natural action: thus blood is the stimulus proper to the heart and arteries; but if, by any accident, it gets into the stomach, it produces sickness or vomiting. The urine does not irritate the tender fabric of the kidneys, ureters or bladder, except in such a degree as to preserve their healthy action: but if it be effused into the cellular membrane, it brings on such a violent action of the vessels of these parts, as to produce gangrene. Such stimuli are called habitual stimuli of parts. Each irritable part differs from the rest in regard to the quantity of irritability which it possesses. This law explains to us the reason of the great diversity which wre observe in the action of various irritable parts: thus the muscles of voluntary motion can remain a long time in a state of action, and, if it be continued as long as possible, another considerable portion of time is required before they regain the irritability they lost; but the heart and arteries have a more short and sudden action, and their state of rest is equally so. The circular muscles of the intestines have also a quick action and short rest. The action of every stimulus is in an inverse ratio to the frequency of its application. A small quantity of spirits, taken into the stomach, increases the action of its muscular coat, and also of its various vessels, so that digestion is thereby facilitated. If the same quantity, however, be taken frequently, it loses its effect. In order to produce the same effect as at first, a larger quantity is necessary ; and hence the origin of dramdrinking. The more the irritability of a part is accumulated, the more that part is disposed to be acted upon. It is on this account that the activity of all animals, while in perfect health, is much livelier in the morning than at any other part of the day; for during the night, the irritability of the whole frame, and especially that of the muscles destined for labor, viz. the muscles for voluntary action, is reaccumulated. The same law explains why digestion goes on more rapidly the first hour after food is swallowed than at any other time; and it also accounts for the great danger that accrues to a famished person upon first taking in food.In German philosophy, irritability, sensibility and reprodudivity constitute the whole of organic life. Since the time of Schelling, irritability is much considered in the mental philosophy of that country. The French, treating the subject merely with reference to physiology, generally use, at present, the word contractility instead of irritability.