CONTRACTILITY. — Since it has been generally understood, that all the most striking and conspicuous movements which take place in living animals, depend on peculiar contrac tions of certain of their solids, the circum stances of these contractions, the causes by which they are excited, and the laws by which they are regulated, have been justly regarded as objects of the highest interest, and of fun damental importance, in physiology. The term Irritability was employed by Haller and his followers, to denote all such contractions in living bodies, as they judged to be peculiar to the living state ; but more recent inquiries have shewn the necessity of distinguishing different species of these contractions; and the more comprehensive term Contractility is now pretty generally employed. To this the epithet Vital, in physiological discussions, may usually be understood as prefixed.
It is to be remembered, however, that several of the animal textures are endowed with a pro perty of contraction, in certain circumstances, which is not peculiar to their living state, but subsists as long as their structure remains unal tered after death ; and the distinction between the phenomena resulting from this cause, and those which are strictly vital, has not always been accurately observed. Thus many of the soft animal textures, muscles to a certain de gree, tendons and ligaments in a greater degree, and arteries in a still greater degree, are elastic, and liable to contractions from that cause when stretched. The Contractilitg de Tissu of Bichat is in most cases to be considered simply as Elasticity, although in some cases (as when he assigns this property as the reason of the re traction of the cut extremities of a living muscle or of the xtiffening of limbs after death,) he gives this name to contractions which are strictly vital. Almost all animal substances are liable to contraction from heat, and from the application of various chemical agents which affect them as astringents, to which property Bichat gave the name of Contractilite par racornissement ; and it is easy to perceive that this property also, although persistent in the perfectly dead body, and therefore inde pendent of life, may give occasion to contrac tions which may sometimes be mistaken for indications of the strictly vital contractility.
Confining our attention, however, to such contractions of the solids of organized bodies, as are exhibited by them only in their living state, i. e. so long as they present that assem blage of phenomena, to which we give the name of Life,--we proceed to state the facts which seem to be most important and best ascertained, first, as to the modes in which they are excited ; secondly; as to their pheno mena, and varieties ; thirdly, as to the condi tions necessary to their manifestation ; and, lastly, as to the laws which regulate them.
I. It is universally known, that the most striking examples of vital contractions are seen in the effects produced by various stimu/i acting on muscles, particularly those of volun tary motion, and the heart. The essential cha racters of muscular fibres, their composition nearly akin to the fibrin of the blood, their arrangement in parallel fasciculi, which are bound together' by cellular membrane, their soft texture, and slight elasticity are also ge nerally known. The change excited by sti muli acting on them is a contraction in the direction of the visible fibres of the muscle, which in the healthy state always rapidly alternates with relaxation; and by these two circumstances,— the excitation by stimulus, and the quickly ensuing relaxation,—we dis tinguish that form of Vital Contractility, to which the term Irritability is most correctly applied.
The stimuli which produce this effect are very various; and the experience of our own bodies points out the obvious distinction of these into physical and mental. Of the first kind, air and water, especially if aided by heat, act decidedly in . this way; but those which have been chiefly used in experiments are, dis tension, especially in the case of the hollow muscles, such as the heart or bladder,—che mical acrids, such as acids, alkalies, various alkaline, earthy, or metallic salts,—and elec tricity or galvanism. The effect of all these stiznuli is much increased by their being sud denly applied.