TONICITY, MUSCULAR. The contractility of muscles shows itself under two distinct forms—irritability and tonicity, which are alike distinct in the mode of their action and in the conditions requisite for their exhibition. Irritability is most manifest in the vol untary muscles and in the heart, which, when in activity, exhibit powerful contrac tions alternating with relaxation; while tonicity is shown in a moderate and permanent contraction, which, instead of being consequent upon stimulation through the nerves, as in irritability, is especially excited by change of temperature in the tissue itself, and is mainly shown in the involuntary or non striated muscles. Like irritability, it is an inherent property of muscular tissue during life. " It manifests itself," says Dr. Car penter, " in the retraction which takes place in the ends of a living muscle when it is divided (as is seen in amputation); this retraction being permanent, and greater than that of a dead muscle. But its effects are much more remarkable in the non-striated
form of muscular fiber; and are particularly evident in the contractile coat of the arteries, causing the almost entire obliteration of their tubes, when they are no longer distended with blood." It is to the moderate action of the tonicity of arteries that their contraction upon the current of blood passing through them is due. If the tonicity be excessive, the pulse is hard and wiry; but if it be deficient, the pulse is very compres sible, though bounding, and the flow of blood is retarded. From the experiments of John Hunter and many subsequent physiologists, it is established that cold is the most efficient agent in inducing tonic contraction; while the application of moderate warmth causes a relaxation of this contraction. Thus, cold and heat arc of extreme value as remedial agents, when the tonicity of the blood-vessels is deficient or excessive.