Neither have I myself, in my treatise on the membranes, sufficiently distinguished these two modifications of contraction, but we evidently ought to establish between them the most decided limits.
An example will render this more sensible. Let us take for it an organ, in which there may be observed all the kinds of contractility, of which I have hitherto spoken; a voluntary muscle for instance: In distinguishing the species with precision we may acquire a clear and precise idea of each of them.
Now such muscle may enter into action first by the influence of the nerves, which it receives from the brain; here it shews its animal contractility. Secondly, it may be brought into action by the stimulus of a physical or chemical agent applied to it, a stimulus, which artificially creates a motion, analogous to that, which is natural to the heart, and other involuntary muscles;—here we have the sensible organic contractility or irritability. Thirdly, its action may be produced by the influx of fluids, which penetrate all its parts for the purpose of carrying thither the matter of nutrition, and which at the same time are the occasion of a partial oscillatory movement in each fibre, in each molecule, a movement as necessary to the function of nutrition, as in the glands it is indispensable to the process of secretion, or in the lymphatics to that of absorption.[37] Such action we refer to the insensible organic contractility or tonicity: Fourthly, by the transverse section of the substance or body of the muscle, may be determined the retraction of its two ends towards their points of insertion. Here the contractility of its texture is displayed.
Any one of these kinds of contractility may cease to exist in a muscle and the others may not be affected. Cut its nerves, and there will be no longer any animal contractility; but the two modifications of its organic contractility will continue to subsist. Impregnate the muscle with opium, suffer its vessels to be well penetrated with this substance and it will cease to contract under the impression of stimuli, it will lose its irritability, but it will continue to possess the tonic movements, which are occasioned by the influx of blood into it. Lastly, kill the animal, or rather let it live, but tie the vessels which go to the limb, and the muscle will in such case lose its tonic power and possess its contractility of texture only. The latter will only cease on the supervention of sphacelus.
By these examples the different kinds of contractility may be appreciated with respect to the organs where they are assembled in a smaller number than in the muscles of volition; in the heart for instance and in the intestines, where there exists a sensible and insensible contractility, the organic being retrenched; and again in the tendons, aponeuroses, and bones, where the animal and sensible organic contractilities are wanting, the insensible organic and the contractility of texture only remaining.
In general these two last are inherent in every kind of organ, the two first belonging to some in particular only; hence for the general character of living parts we must choose the insensible organic contractility or tonicity, and for the character of all organized parts whatsoever, whether living or dead, the contractility of texture.[38]
We shall farther remark, that this last in the same way as its corresponding extensibility possesses them, has its different degrees, its scale of intensity, the skin and the cellular substance on the one hand, the tendons, the aponeuroses, and the bones on the other, forming the extremes of this scale.
From all that has been said, it is easy to perceive, that in the contractility of every organ there are two things to be considered, namely the contractility, or the faculty, and the cause, which puts it in action. The contractility is always the same, belongs to the organ, is inherent in it, but the cause which determines its exercise may be various.