In order to understand what part the abdominal muscles by their contractions take in vomiting, we ought to observe what takes place when these muscles are unable to act. There is but one way of coming at this, which is, to separate these muscles from their attachments at the sides of the linea alba; this we have done on many animals; we have detached successively the external oblique, the internal oblique and the transversalis, leaving on the anterior face of the abdomen only the peritoneum. When these muscles are thus removed, we can see very distinctly through the peritoneum, all that takes place in this cavity; we distinguish, for example, perfectly the peristaltic motion of the stomach and the intestines; and if the stomach contracts it will be easy to see it. The abdominal muscles being thus detached, I injected three grains of emetic into the jugular vein, and also immediately nausea and vomiting took place by the contraction of the diaphragm alone. It was curious to see, in the convulsive contraction of this muscle, the whole intestinal mass pushed downwards, and pressing strongly against the peritoneum, which was ruptured in some places. In this case, the linea alba, formed by a very strong fibrous texture, is the only part which resists the pressure of the viscera; its existence then is indispensable to the action of vomiting; perhaps it performs an analogous office in the ordinary state. This experiment proves that vomiting can be produced by the efforts of the diaphragm alone; this is also confirmed by the following experiment:
I detached, as above, the abdominal muscles and laid bare the peritoneum; I afterwards divided the diaphragmatic nerves, and injected an emetic into the veins. The animal had some nausea, but nothing more. Though I repeated many times the injection of the emetic, I never was able to produce any sensible effort of vomiting.
From the different experiments that we have just related, and from the facts that we made known in a preceding note relative to the motions of the œsophagus, we may conclude, without any hazard,
1st. That vomiting can take place without any contraction of the stomach.
2d. That the pressure exerted immediately on the stomach by the diaphragm and abdominal muscles, appears to be sufficient to produce vomiting, when the occlusion of the inferior part of the œsophagus offers no obstacle to it.
3d. That the convulsive contraction of the diaphragm and abdominal muscles, in vomiting from tartarized antimony and emetic substances properly so called, is the result of a direct action of these substances on the nervous system and independent of the impression felt by the stomach.
[35] The motions of the iris cannot be attributed to an active expansion of an erectile texture; they are owing to the contractions of two muscular layers, one of which is radiated and enlarges the opening of the pupil, the other is orbicular and contracts it.
The motions of the iris, like all those which have muscular contraction for their cause, can be excited for a considerable time after death by the galvanic fluid. During life, the motions of the pupil are produced in man, by the more or less vivid impression of light on the retina. But they are beyond the influence of the will; in birds on the contrary, they appear to be entirely subjected to it. In these animals, we can even after death, and on an eye entirely detached from the body, produce the motions of the iris by pricking the optic nerve.
[36] When a patient dies after having for a long time been deprived of solid and liquid nourishment, it is not rare to find in him the stomach and intestines considerably lessened in their two dimensions, the internal cavity almost entirely effaced, the length being hardly a third of what it was before the disease. We truly say then with Bichat that is a contraction from a want of extension. But that this mode of contractility is as he says perfectly independent of life and owing only to the arrangement of parts, is what cannot be admitted. If it were so in fact, by emptying the stomach after death, we might produce a contraction similar to that which is produced during life. Now experiment shows us, that this does not take place. The stomach when emptied remains flaccid, and does not contract in any perceptible degree.
[37] We know that the organs are nourished, that the glands secrete, we know that certain vessels absorb (whether they be the lymphatics or not,) but we do not know, that all this is produced by a partial oscillatory movement in each fibre, in each molecule. No one can be certain that this movement takes place, because no one has seen it.