We have seen that the voluntary muscles have in general a degree of quickness beyond which they cannot go, and that this quickness belongs to the original constitution. Is not the same phenomenon observed here? Often in two fevers whose symptoms are the same, whose degree of intensity seems to be exactly uniform, the pulse is infinitely more frequent in one individual than in the other. This does not always denote a difference in the disease, but in the primitive constitution, an aptitude of one of the two hearts to contract much quicker under the same stimulant. Who does not know that in experiments, the contractile rapidity is infinitely variable under the influence of the same causes?
Each organic muscle has its degree of quickness; the heart, the stomach, the intestines, the bladder, &c. differ remarkably in this respect.
III. Duration of the Contractions.
The heart never remains in permanent contraction, as often happens in the voluntary muscles. Though hunger seems to prove the contrary in the stomach and the intestines, yet this phenomenon is not contradictory; in fact, the permanent contraction of the empty gastric viscera is the result of the contractility of texture. Whenever the sensible organic contractility is in action, there is alternate contraction and dilatation; this alternation even characterizes essentially this last property, and distinguishes it from the animal contractility and from that of texture, in which the state of contraction is often permanent.
IV. State of the Muscle in Contraction.
All the phenomena described for the voluntary muscles, are almost applicable to these, such as the hardening, increase in thickness, diminution in length, expression of the blood, &c. &c. But there are some differences between the heart and the gastric muscles, in respect to the mode of contraction. In fact we see very sensibly in the first, 1st, contractions of the whole analogous to those of the voluntary muscles, contractions which take place in the state of health, which produce the projection of the blood, and which are easily made in experiments when the animals are still living; 2d, numerous oscillations which seize upon the fibres, which agitate the whole of them without producing any sensible effect, without contracting the cavity, without projecting the blood for example. These oscillations are observed at the instant of death, when the heart is ceasing to be contractile; we may then irritate it in vain, there are no more contractions of it as a whole; though there is a general and very evident vibration of its fibres, yet its cavity is not contracted; the blood stagnates in it. The heart perfectly resembles under this double relation the voluntary muscles; it is agitated as we see these muscles in the shuddering, that is called horripilatio, as we see it also in certain sub-cutaneous muscles in some individuals. I have already, for example, seen many persons affected with an habitual trembling of a portion of the solæus, a trembling very evident to the eye through the skin, and which had nothing in common with the contraction necessary to the extension of the foot.
The involuntary muscles of the abdomen never exhibit this double mode of contraction. Instead of the quick and sudden motions of the whole of the muscle, we see but a slow contraction in it, often but slightly apparent; it is a kind of creeping; there is not even to speak properly a contraction of the whole, like that of the heart in which all the fibres of an auricle or ventricle are moved at the same time; here each fleshy surface appears to act successively. Placed at the origin of the great vessels, the bladder and the stomach would be incapable of communicating to the blood those motions by jerks, which the jet of an artery exhibits at each contraction. On the other hand, at the instant the motion ends in the stomach, the intestines and the bladder, we never see in them those oscillations, those vibrations which are almost constant in the heart and the voluntary muscles, and which we can even create in them at will.
V. Motions imparted by the Organic Muscles.
There are hardly ever simple motions in these muscles; the different interlacing of their fleshy surface allows them to act almost always in three or four different directions upon the substances they contain. We can say nothing general upon those motions which compose the diastole of the heart, the peristaltic motion of the alimentary canal, the contraction of the bladder, &c. Each muscle has its mechanism which belongs to the physiological history of the function to which it contributes.