Caloric, by its absence which constitutes cold, as by its presence from which arises heat, can equally excite the muscles and in general all the organs. At the instant we open the thorax or the pericardium of a living animal, the heart is agitated with a suddenly increased force; it is because the air acts upon it, and it passes from the temperature of the body to another which is different. All the aeriform fluids, light, all fluids, &c. are stimuli of the muscles. If we see the heart emptied of blood, the stomach and intestines deprived of the substances that ordinarily enter them, contract with more or less force when they have been taken out of the body, it is because the surrounding medium, and the substances with which it is charged, contribute to produce this effect; they are then the stimuli of these organs.
In general the artificial stimuli act in different ways; 1st, by their simple contact; 2d, by tearing or cutting mechanically the fibres; 3d, by tending to combine with them; 4th, there are some of whose mode of action we are completely ignorant; such for example is electricity.
When the stimuli act only by simple contact, the fluids are, all things being equal, more efficacious than the solids, because they stimulate by a greater number of points; as they irritate not only the surfaces of the organ, but penetrate also into the interstices of the fibres. The solids produce an effect in proportion to the extent of their excitement, to the greater or less pressure that they exert, to their density, their softness, &c. They are almost always fluid substances that nature employs for stimuli in the ordinary state.
Tearing is a mode of excitement more active than contact. The heart, the intestines often inert when they are only touched by the scalpel, contract powerfully when the point of it excites them. Cutting produces a less sensible effect than tearing. Cut transversely, the fibres oscillate and are agitated only by the sensible organic contractility, whilst by the contractility of texture they experience an evident retraction.
Chemical excitement is, in the greatest number of cases, the most advantageous; but it is necessary here to distinguish that which belongs to the horny hardening, from that which is the effect of irritability brought into action. 1st. Plunge a frog without skin and alive into a concentrated acid; instantly every thing is disorganized; the reagent acts so strongly, that we can distinguish neither horny hardening nor contractility. 2d. Weaken the acid a little and plunge into it, the inferior extremities only of a frog; in an instant they stiffen by the contraction of the extensors, which overcome the flexors; for in this experiment, this is almost a constant phenomenon; withdraw the animal; its thighs remain immoveable, life has been extinguished in them; the contraction that has come on is a horny hardening, and not a vital phenomenon. A dead frog plunged into the same liquor experiences the same phenomenon. 3d. Weaken the acid still more; the instant the animal is plunged into it its limbs contract; but relaxation succeeds the contractions; these are alternate motions; it is the irritability that begins to be put into action. Yet if the acid is not very weak, some marks of the horny hardening still remain, and the animal has a stiffness in the motions of the inferior extremities, the evident result of the first degree of this horny hardening. 4th. Finally, if the acid is very weak, it becomes a simple irritant which puts in action the sensible organic contractility, without altering the texture of the fibres; the animal after coming out of the fluid preserves the same power of motion.
These experiments which it would be easy to multiply upon animals with warm blood, but which I have never attempted upon them, evidently show what belongs to the horny hardening, and what is the effect of vital contraction. Yet there is not an exact limit between them, and there is one degree of weakness of the acid in which these two causes of motions are confounded.
There is a mode of excitement to which authors have not paid attention; it may be called negative; it is that of which I spoke just now on the subject of caloric, the privation of which is oftentimes a very active stimulant. In the different experiments that I have had occasion to make, this has frequently struck me. Apply a stimulant to a muscle, it contracts; but at the end of some time the motion ceases, though the contact continues; remove the stimulant, the motion frequently returns in an instant. In general, nothing is more common in the heart, the intestines, &c. than their contractions ceasing under the continued action of a stimulant, and returning instantly upon its absence. I confess that this phenomenon is not as invariable and constant as that of the contraction produced by the application of the stimulus which succeeds a state of non-excitement; but this happens very often. We might say that the organic sensibility is in this case like the animal, that every new state affects it, whether it be positive or negative. The passage from non-excitement to excitement is more lively; but the opposite passage is not less when it is sudden. Moreover this manner of describing the sensible organic contractility in exercise, deserves some further experiments.
Of the Sensible Organic Contractility considered in relation to the Organs.
The sensible organic contractility, considered in the organ in which it has its seat, exhibits numerous varieties which are relative; 1st, to the diversity of texture; 2d, to age; 3d, to sex; 4th, to temperament, &c.