These aberrations of animal sensibility then exist entirely in the brain; it is an irregularity, a derangement of perception; this irregularity presents phenomena analogous to the following; we often refer to the skin a sensation of heat, as we shall see, though caloric is not disengaged there in a greater quantity than usual. We know that oftentimes the sensations of hunger and that of thirst are purely sympathetic, and that the cause which produces them in a natural state does not then exist in the stomach or intestines. We know the illusions of vision, of hearing, of the smell even, &c. We have not studied sufficiently the irregularities of perception; those of the memory, the imagination, the judgment, &c. have been analysed. These, however, have been almost forgotten. They perform the greatest part in animal sensibility.
2d. Animal contractility supposes constantly nervous action, when it is put in exercise sympathetically. In fact we shall see that this property cannot be exerted without the triple action of the brain, of the nerves that go to the muscles that move, and of the muscles themselves. When a muscle of animal life is brought into action by the irritation of any distant organ, by the distension of the ligaments of the foot for example, this organ acts at first upon the brain, which then reacts by means of the nerves upon the voluntary muscles that are concerned in convulsions. The following is an experiment by which I satisfied myself of the cerebral and nervous influence in the sympathies that occupy us. I cut all the nerves of the inferior limb of one side, in different animals, and I afterwards irritated in a thousand different ways, very irritable parts, as the retina, the pituitary membrane, the marrow of the bones, &c. I produced in this way a number of sympathetic phenomena, sometimes of organic contractility, as vomiting, involuntary evacuations of urine, fecal matter, &c. sometimes of animal contractility in the muscles whose nerves remained untouched. But the muscles whose nerves were cut, were never brought into action. I have very frequently repeated these experiments, which would have certainly produced results, if the nervous communications could, without the intervention of the brain, make the muscles of animal life contract. I would observe upon this subject, that sufficient regard has not been paid in experiments upon sensibility, to the sympathetic phenomena. I do not know even that these phenomena have been the object of any experiments upon animals, before those of which I have here given the first results, and which I propose to multiply under other points of view. There are then two things in all sympathy of animal contractility, viz. 1st, the action upon the brain of the organ that suffers, by means, of which as yet we know but little; 2d, reaction of the brain upon the voluntary muscles. In this last period of sympathy, the nerves of animal life are the agents constantly necessary.
3d. The cerebral nerves and brain have evidently no connexion with the sympathies that put in action sensible organic contractility or irritability. If they had, the affected organ would first act upon the brain, and this would react upon the involuntary muscle; thus, when tickling produces vomiting, there would be an action of the skin upon the brain, and of the brain upon the stomach. Now the brain never exerts any influence upon the involuntary muscles; whatever be the irritation that the nerves experience which go to them, the muscles remain unaffected. Then although the brain may be sympathetically affected, it does not react upon the involuntary muscles; the cerebral nerves then have no connexion with the sympathies of sensible organic contractility. The continuity of the membranes is not a more substantial cause, and this is the proof of it. We know that by irritating the uvula, the stomach heaves; now as the mucous surface of the one and the other is the same, we might attribute this sympathetic phenomenon to this circumstance. I have then made a wound in the side of the neck of a dog; taken hold of the œsophagus and cut it transversely; the uvula has been afterwards irritated; the dog, notwithstanding the interruption of continuity, made efforts to vomit as before. Let us acknowledge then that we do not know the cause of the sympathies of sensible organic contractility.
4th. As much may be said of the sympathies of organic sensibility and insensible contractility. We have proved that the nerves have no influence upon these two properties; that by acting upon them we neither increase or diminish them in any manner, and that their diseases do not disturb the functions over which these properties preside. Then when they are sympathetically disordered, the nerves appear to have no connexion with these phenomena. Thus, 1st, every sympathetic exhalation, as the sweats of phthisical patients, certain serous infiltrations that take place almost instantaneously, &c.; 2d, all secretions of the same kind, as those which appear in a number of diseases afford us examples of them, &c.; 3d, all analogous absorption, the three functions over which the preceding properties preside, are evidently unconnected with the nervous influence of animal life. I shall say the same of the cellular, vascular influences, &c. Certainly we have no data, by which we can explain how these means of communication produce sweat when the lungs are affected, and saliva in the mouth when the membrane of the palate is irritated, &c.
From all that has been said it follows, 1st. that the sympathies of animal sensibility appear to be in the greatest number of cases an aberration of the principle that perceives in us, and which is deceived as to the place in which the causes of sensation act; 2d. that the sympathies of animal contractility require inevitably the intervention of the brain, but we know not how the part affected acts upon this viscus, though we know very well how this viscus sympathetically excited reacts upon the muscles to make them contract; 3d. that the causes of the two kinds of organic sympathies are absolutely unknown and that a thick veil hides the agents of communication which connects, in this case, the organ from which the sympathetic influence goes to that which receives it.
It is this obscurity of the sympathetic causes, that has made me entirely neglect every kind of hypothetical opinion, in classing the sympathies in this work, in which I examine them in each system of organs. I have had regard only to a natural division, to that indicated by the vital forces of which the sympathies are but an irregular exercise. Now by limiting ourselves to the most rigorous observation, it is evident that this division is the only one that is admissible; and I believe that there is no other to be employed, until our knowledge shall be sufficiently extended to admit of their being classed by the causes that produce them, and not by the results they present.
Besides I cannot recommend too strongly the necessity of distinguishing what belongs to them from that which arises from the natural connexion of functions. Observe what takes place in syncope, apoplexy and asphyxia; one organ is disordered; all the others soon cease to act. Sympathies have no part in these phenomena. Physicians have been much embarrassed by classing these affections, sometimes as if they belonged to the nerves, at others to the sanguineous system, &c. This is what takes place in each.
1st. The heart first ceases to act in all syncopes, whether they arise from passions of the mind, disagreeable odours, &c. The circulation being stopt, the brain is no longer excited by the blood; it ceases its action, and the whole of the animal life is interrupted. The organic life that the blood supports, is thus suddenly annihilated. 2d. Asphyxia commences in the lungs. Respiration is deranged; it sends to the brain blood that cannot excite it; this ceases to correspond with the senses, to determine involuntary motions, &c. &c. 3d. It is in the brain that apoplexy has its first seat; thus animal life is immediately interrupted; then, when it is very severe, the brain not being able longer to support the motions of the intercostal muscles, these motions are stopt; the mechanical, then the chemical action of the lungs ceases; circulation cannot go on, and organic life is interrupted. We see then, that in all the phenomena of these affections, the injury of one organ, produces, by a natural consequence, the suspension of the action of the others.
This is wholly different in the sympathies. Thus the functions of the skin being suspended, sometimes the lungs, sometimes the stomach, and sometimes the intestines, feel it and are affected by it; these sympathetic phenomena may manifest themselves or may not; on the contrary, whether it be the cerebral, pulmonary or cardiac action, that is deranged, it is impossible but that the others should be consequently affected.