The animal sensibility of the nerves has a peculiar character which distinguishes it from that of all the other systems. It is this character which gives a peculiarity to the pain in these organs, which does not resemble that which has its seat in the skin, in the mucous surfaces, &c. What particularly fixed my attention upon the difference of pain of which each system is the seat, was the question of a man of great mind and coolness, whose thigh was amputated by Desault; he asked me, why the pain he felt when the skin was cut, was wholly different from that which he experienced when the flesh was cut through in which the nerves, scattered here and there, were divided by the knife, and why this last sensation differed entirely from that which was felt when the marrow was divided. This embarrassed me then, when I was wholly engrossed in surgery, and had studied physiology but little; I have seen since, however, that it is to be referred to that general principle of which I have already spoken, and which determines, that as each system has its peculiar kind of animal sensibility in a natural state, it has it also in a morbid state, that is to say, in pain.
A very clear proof of this assertion, as it regards the nerves of animal life, is the peculiar kind of pain that is experienced in the tic douloureux, a kind that is unlike that of any other system. The sciatic disease, which has its seat in the nerve of the same name, has often been confounded with rheumatism, which affects the muscles or tendinous parts; but the difference of pain alone is sufficient to distinguish them. Mr. Chaussier has very judiciously taken for the first character of neuralgia, the nature of the pain. Every one knows the peculiar sensation of numbness and afterwards of pricking, that is felt when a superficial nerve, as the cubital, &c. is compressed. No other organ in the economy gives the same sensation from the same cause.
The animal sensibility of the nerves has another peculiar character, which consists in this, that the local irritation of a trunk often produces suffering in the whole branches. 1st. We know that when the cubital is compressed at the elbow, the pain extends along its whole course, and that it spreads over the whole external part of the leg, when the peroneal suffers. 2d. In the tic douloureux of the face, in the sciatic disease, and generally in all that class of diseases of which Mr. Chaussier has given a sketch under the name of neuralgia, an analogous observation may be made. 3d. When we wound, without dividing, one of the branches of the saphena, the internal cutaneous or muscular cutaneous, in the operation of blood-letting, the subjacent part frequently becomes numb, then painful and swelled; the irritated point is a centre, whence go forth, along the whole course of the nerves, painful irradiations, the consequences of which oftentimes cannot wholly be stopped, except by dividing the irritated trunk. Thus, in tic douloureux, the division of the nerves has sometimes overcome the disease, though we shall succeed less frequently here by these means than in the preceding case, where the affection is local, while here it is usually extended along the whole course of the nerve. 4th. I have irritated, in a dog, the sciatic nerve with nitric acid; the whole limb was swelled and painful the next day. I have at this time another, the whole of whose fore limb is swelled, because I passed a pin, two days before, through one of the anterior nerves, taking care to entangle some of the nervous filaments. This precaution is essential, for I passed a pin through the cellular texture that separates the filaments of the sciatic, without producing any effect. I should observe, however, that these different experiments do not always succeed, and that I have irritated a nerve at one point sometimes without producing any effect. 5th. The ligature of nerves is rarely followed by these accidents, because the communication with the brain is interrupted, by the very means that irritate, and because the medullary substance is flattened and its sensibility destroyed. However accidents have often happened from tying a nerve in the operation for aneurism, and though there is no real danger in making the ligature, all good practitioners advise that it should be avoided.
These different considerations prove in a positive manner, the influence that a portion of an irritated nerve has upon the animal sensibility of all the subjacent ramifications. Physicians do not give sufficient attention to this cause of pain, which is often very extensive without any apparent wound. An irritated nerve in a fracture of the ribs, in that of a limb, in a wound, in a tumour, &c. can produce at a distance a number of phenomena, the cause of which often escapes us, and which we should soon discover if we reflected upon the distribution of the branches going from the trunk of the nerve that is near the affected part.
Why in these phenomena, is the animal sensibility of the nerve below the affected part always raised? Why does this phenomenon never take place on the side of the brain, though it is in this direction that sensation is conveyed in a natural state? I know not.
No other system, among those all of whose parts are united like the nervous system, presents the same phenomenon. The arterial, the venous, the absorbent, never feel thus in their different ramifications, the affections of any one part of their trunk. The cellular is not affected at a distance by the diseases of one of its parts. In the mucous which is continuous, a part being irritated, oftentimes others also are affected, as when the stone in the bladder produces suffering in the glans penis; but there is always an intermediate portion more or less considerable, which remains without being painful; this is a real sympathy; whereas in the other, the whole nervous trunk suffers, from the affected part to the nervous extremities.
After having considered the animal sensibility in the nervous system itself, we must examine the part this system performs in this property described in relation to all the other organs, in which it is often the means of transmission between the organ that receives the sensation and the brain which perceives it. So that when any point of the nervous system suffers, as in the preceding cases, the portion of nerve that is between this point and the brain, serves to conduct the impression. Thus in animal contractility, the nerves are always intermediate to the brain, which is the principle of the motion, and to the muscle that executes the motion. There is, however, more difficulty in the first kind of transmission than in this, which, to be explained accurately, requires that we should distinguish two kinds of sensations perceived by the internal sensitive principle, 1st. the external; 2d. the internal.
The external sensations are of two orders, 1st. the general; 2d. the particular. The general sensations are derived from the sense of feeling, as we shall see; they indicate the presence of the bodies that are in contact with the external organs; they give the general impressions of heat and cold, moisture and dryness, hardness and softness, &c.; they produce a painful sensation when the external organs are torn, pricked, or acted upon by chemical agents, &c. These sensations may originate upon the skin, the eye, the ear, the mouth, the nostrils, upon the beginning of all the mucous surfaces, &c.; all the bodies in nature may produce them, and all the external organs may perceive them. 2d. The particular sensations are relative to certain determinate external bodies, or to particular emanations from surrounding bodies. Thus the eye exclusively perceives the light, the nose odours, the ear sounds, the tongue tastes, &c. These particular sensations are to a certain degree independent of the general ones; thus the eye may cease to see, the nose to smell, the ear to hear, the tongue to taste, and yet these different organs may preserve the faculty of perceiving the general attributes of heat and cold, moisture and dryness, &c. and may be the seat of real pain. Every day we see patients affected with gutta serena suffering from the eye, those affected with deafness having pains in the ear, &c. I have seen a man that was deprived of the sense of smell from the use of mercury, and who still would suffer very much if the pituitary membrane was irritated, &c. It is necessary, then, to distinguish in the organs of sense, that which belongs to the general sense of feeling, from that which is dependant upon the particular kind of feeling that each has separately.
If now we examine the part of the cerebral nerves in these two kinds of animal sensibility, it appears that they are equally essential to one and the other. 1st. This is without doubt as it regards the organs of sense; the sight, the hearing, the smell, or the taste, could never continue after a serious injury of the optic, auditory, olfactory, gustatory nerves, &c. I do not speak of the touch, which does not require, like the other senses, a peculiar modification of animal sensibility, but only the general feeling, with a peculiar form in the organs that are provided with it, so as to mould themselves to the figure of external bodies. 2d. As to the general sensations, whenever the cutaneous nerves cease entirely to act in any part of the skin, it becomes absolutely insensible; it may be pinched, irritated, burnt, &c. without feeling it. The perfect paralyses of sensation exhibit in man this phenomenon, which can easily be produced in animals by cutting or tying all the nerves that go to a limb. When the general feeling is left in the pituitary membrane after the loss of smell, the olfactory nerve is alone paralyzed; if the nerves that enter by the spheno-palatine foramen, through the anterior and posterior openings of the nostrils, cease also to act, then the general feeling is likewise lost. It is the same with regard to the other organs of sense.
I believe, then, that the nerves are actually necessary to the external sensations, whatever be their nature. Observe also, that all the organs with which external bodies can be in contact, as the dermoid system, all the origins of the mucous systems, and the organs of sense, are provided more or less abundantly with cerebral nerves; none of them receive the nerves of the ganglions. This external portion of the nervous system of animal life is very considerable; united to the portion that goes to the voluntary muscles, it forms almost the whole of this system, which has but very few appendices in the organs of internal life.
As to the internal sensations, they have phenomena much more obscure than the preceding. The brain is undoubtedly the centre of these sensations, as well as those which take place without it; in fact, if the action of this organ is suspended by wine, opium, or any other means, though acute pains may affect the internal organs, these pains are not perceived. Thus when the brain has received a concussion, though the impression of sounds, of light, of odours is made as usual upon the ear, the eye and the nostrils which are uninjured, yet there is neither hearing, seeing or smelling. But how do the impressions made upon the internal organs get to the brain? Here are different phenomena, that it is impossible to conceive of well, by supposing that the nerves are charged with transmitting these impressions exactly like those which are experienced by the external organs.
1st. There are organs that have the most acute sensibility upon the slightest touch, and which however receive very few apparent nerves; such is the medullary membrane of the long bones. 2d. Certain organs in which the cerebral nerves evidently enter, as the liver, the lungs, &c. can be irritated in animals, without seeming to give them much pain. 3d. The muscles of animal life, in the structure of which so many nerves enter, in which also the branches of these perform so great a part as it relates to animal contractility, do not occasion much pain when their texture is cut without entangling the nervous filaments that penetrate them. 4th. The ligaments, that no nerve enters, are the seat of acute pain when they are distended, as my experiments have proved. It is the same as it respects the tendons, the aponeuroses, &c. 5th. All the organs with the structure of which the nervous system has manifestly no connexion, transmit however to the brain the most painful impressions when they are inflamed, &c. &c.
I could bring many other facts, which the opponents of Haller have carefully collected; but these are such, that we cannot hesitate to admit, that the opinion of this celebrated physiologist should not be entirely acceded to.
All that we know upon the internal sensations is, that, 1st. there is an organ in which the cause of sensation is seated; 2d. that this organ transmits to the brain the particular modifications that it experiences in its vital forces. But we are wholly ignorant of the medium of communication of one with the other. Hence, why, in my division of the vital forces, I have avoided a systematic basis. The distinction of the two kinds of sensibility, of the three kinds of contractility, rests wholly upon the observation of facts. Such is the obscurity of the phenomena of life, that I doubt if we shall ever be able to establish divisions from a knowledge of the nature and essence of the vital forces.
I observe that there is a great difference between animal sensibility and contractility; that in the first, the nerves are in certain cases the evident agents of communication between the organs that receive the impression and the brain that perceives it, but that in other cases, we know not the kind of relation; whilst in the second, it is always manifestly by the nerves that the brain communicates with the muscles, and that the organs can never execute a voluntary motion without the influence of the cerebral nerves.
Let us confine ourselves to this general view, which is from accurate observation; let us abandon reasoning, where experiments are not the basis of it. Some modern authors have been less judicious; they have admitted a nervous atmosphere extending more or less remotely, and acting at a determinate distance; so that though an organ may not receive a nerve, it is sufficient that it should be in the atmosphere of a nervous cord to be the seat of sensations. This ingenious idea of Reil, should be placed at the side of a great number of those that Bordeu has scattered in his works, and which are rather proofs of an ingenious author, than an accurate and judicious mind, hostile to every opinion not founded on rigorous experiment. In fact, what is this atmosphere? Is it an emanation that is constantly made at the exterior of the nerves? Is it a fluid that is independent of them, and that nature has placed around each nervous cord, as it has placed the air around the earth? Is it a power that has been given to the nerves to act at a distance without intermediate bodies? Some galvanic experiments seem to prove something similar to this in the nerves; but these experiments have no relation to the transmission of animal sensibility. Moreover, when pain takes place in the middle of a very thick tendon, in the centre of a large articulation, as that of the knee for example, it would be necessary that the atmosphere of nervous activity should extend sometimes even an inch. Why is there not suffering produced, by the irritation of an insensible part that is at the side of a nerve, or even connected with it, whilst the pain is very acute in an inflamed part, though it is at a distance from every nervous cord? Would the nerves then have also, a sphere of activity for motion? But why should the contiguity of the nerve never be sufficient to produce it in the muscles? Why is it not the same of sensation?
The texture of the nerves is wholly destitute of this contractility. No kind of sensible motion is ever observed in them; they perform however an essential part in this property, considered in relation to the muscles of animal life. We shall see that they are the essential agents which transmit to them the principle of motion; so that animal contractility always supposes the exercise of three successive actions, viz. that of the brain, the nerves and the muscles.
The opinions of physiologists have been singularly divided upon the manner in which the nervous influence is propagated. Some have admitted a kind of vibration, others a fluid pervading the insensible canals of these organs. This last hypothesis is still in much credit. What has not been said upon the albuminous, electric, magnetic nature, &c. of this fluid? The article upon the nerves, in most physiological treatises, is almost wholly devoted to the examination of this question, but I shall say nothing upon it, for we do not know any thing that rests upon experiment. Moreover, are we not able without knowing the mode of the nervous action, to study and analyze the phenomena of the nerves? It is the common fault of all the ancient physiologists, to have wished to begin where they should one day end. Science was in its infancy when all the questions they discussed turned upon the first causes of the vital phenomena. What is the result of it? An immense deal of rubbish, and the necessity of finally coming to the accurate study of these phenomena by abandoning that of their causes, until we have observed enough to establish theories. Thus mankind have disputed for ages, upon the nature of fire, of light, of heat, of cold, &c. until philosophers finally perceived that before reasoning it was necessary to have a foundation upon which their reasoning should rest, they then sought for these foundations and thus created experimental philosophy. Thus interminable disputes have existed in the schools upon the nature of the soul, of judgment, &c. until metaphysicians have perceived the necessity of analyzing the operations of our intellectual faculties before they can know their essence. Each of the natural sciences has almost had two epochs; 1st. that of the last age, in which first causes were the only subject of discussion; an epoch useless to the sciences; 2d. that in which they have begun to be composed of the study of the phenomena only that experience and observation offer. Physiology has still one foot in the first epoch, whilst it has placed the other in the second. Physiologists of the present day should advance it still further.
They are in general very indistinctly marked in these organs. They want sensible organic contractility. The insensible and the organic sensibility are there only to that degree that is necessary to nutrition; for these properties have no other functions to support there. Thus observe, that almost all the diseases of the nervous system are affections of the animal sensibility, and that but very few suppose a disorder in the organic. There is hardly ever an alteration in the nervous texture; no tumours, fungi, ulcerations, &c. as in the systems in which the organic properties are predominant. Thus morbid anatomy finds but little to exercise itself upon in the nerves.
The continual motion of a part sometimes increases a little the organic sensibility of the nerves that are found there, makes their nutrition more active and their size more apparent; but generally this phenomenon is infinitely less sensible in them than in the muscles. On the other hand though the nerves may have lost the faculty of transmitting sensation and motion, this last especially, they still preserve for a long time the same degree of organic sensibility, and their nutrition is the same as usual. I have many times examined comparatively the nerves of the sound side and those of the side affected with hemiplegia; I have never found in them any difference. It is only when the limb becomes atrophous, which never happens except at the end of a long time, that the nerve diminishes in size.
I have often searched to see, if, when a part, in which there are nerves, has been a long time the seat of uninterrupted painful sensations, the nutrition of these is altered, and consequently if their organic sensibility is affected. I have dissected the stomachic cords in cancers of the pylorus, the uterine nerves in those of the womb; I have not found any sensible difference, except in two subjects, in whom they were a little enlarged. Desault discovered also in a body affected with carcinoma of the fingers, the median nerve of uncommon size; but this phenomenon certainly is not as general, as the dilatation of the arteries in this kind of tumours. As to acute pains, like those of rheumatism, of different inflammations, &c. however severe they may be, they have no effect upon the nutrition of the nerves which transmit them. When the pain is seated even in the nervous texture itself, as in the tic douloureux, there is oftentimes no organic affection. At least Desault had occasion to open two patients that had had that disease, and the nerves were the same on each side. This, however, deserves further research, and it may be, that in many cases, the internal substance of the nervous cords is a little altered; for I preserved the sciatic nerve of a patient, who had experienced very acute pain in its whole course, and this had at its superior part, a number of little varicose dilatations of veins that entered it.
Have the cerebral nerves any influence upon the organic sensibility of other parts? I think not, and this is the essential difference that distinguishes it from animal sensibility, which we can with difficulty conceive of, especially in its natural state and in the external sensations, without the nervous influence intermediate between the brain and the part that receives the impression. To prove this assertion, let us examine the functions that depend upon organic sensibility. These are, 1st, the capillary circulation, 2d, secretion, 3d, exhalation, 4th, absorption, 5th, nutrition. In all the phenomena of these functions, the fluids make an impression on the solids of which we have no consciousness, and in consequence of which the solids react. It is by the organic sensibility that the solid receives the impression, it is by the insensible contractility that it reacts; now in none of these cases do the nerves appear to perform an essential part.
1st. The capillary circulation exists in the cartilages, the tendons, the ligaments, &c. in which the nerves of animal life do not enter. Inflammation, which is only a derangement, an increase of this capillary circulation, takes place in these organs, as well as in those that are the most nervous; what do I say? where the nerves are the most numerous, this affection is not the most frequent; the muscles are an example of this. The tongue, whose surface alone has more than four or even five times the quantity of nerves of the mucous surface, is not so often inflamed as the rest of this system. The retina, which is entirely nervous, is rarely inflamed. Nothing is more rare as I have said, than the inflammation of the nerves themselves; the internal substance of the brain is hardly ever inflamed. On the other hand, examine the serous surfaces, the cellular texture, in which there are infinitely less nerves; the capillary circulation is constantly active there, and inflammation comes on. In the limbs of paralytic patients, in animals in whom the nerves are cut in order to render a part insensible, does not the capillary circulation continue as usual, when the nervous action has ceased there? Have you ever accelerated this circulation in a limb, or produced inflammation, by increasing convulsively by irritation the action of the nerves of this limb? The phenomena of convulsions and those of paralysis, are wholly distinct and have no analogy with those of inflammation; this would not be so, however, if the cerebral nerves had any influence upon them. In the first phenomena, it is the animal sensibility that is altered; in the second it is the organic; this is then independent of the cerebral nerves.
2d. Exhalation is the second function over which this last property presides. I refer to the dermoid system to prove that the sweat is independent of the nerves. I would only observe here, that in the synovial, in which there is an evident exhalation, there are hardly any nerves; that the serous surfaces and the cellular texture, so remarkable for this function, are, as I have said, almost destitute of them; that whenever there are accidental exhalations, as in cysts, hydatids, &c. the nerves have evidently no influence, as the tumour is uniformly without them; that by acting in any manner upon the nervous system, as by irritating the nerves, the brain, or the spinal marrow, in order to excite this system, as by tying or cutting the first, and compressing the second, to annihilate or weaken its action, the cellular, serous, synovial, or cutaneous exhalations are never in any way affected; and that finally the diseases of the nervous system have no other influence upon this function, than what is derived from general sympathy.
3d. As much may be said of absorption. It is during sleep that the skin oftentimes absorbs most easily; now there is at that time, an intermission in the action of the nervous system, as well as of that of the brain. This intermission, to which it is periodically subject, ought to produce one in the serous, synovial, medullary absorptions, &c.; but yet they go on constantly. It is the same of all the functions over which the organic sensibility presides; they are essentially uninterrupted, though the nervous and cerebral actions are essentially intermittent.
4th. The same observation may be made concerning the secretions, notwithstanding what Bordeu may have said. For the rest, I refer upon this point to the glandular system.
5th. Nutrition takes place in parts that evidently receive no nerves, as in the cartilages, the tendons, &c.; in paralyzed limbs also its alterations are always independent of those of the nervous system. Those people in whom this system is the most elevated, who are the most sensitive, are not those in whom nutrition is the most active. In no experiment, I believe, has any one been able to influence nutrition by acting upon the brain, the nerves, and the spinal marrow. Marasmus undoubtedly succeeds all prolonged nervous diseases; but it is a phenomenon common to many diseases. In palsy, the long rest, as well as the deficiency in the action of the nerves, has an influence in producing atrophy; but it is a long time before this manifests itself. Who does not know that often at the end of two, three, or four years even, the diseased limb is exactly of the size of the well one? Moreover, natural nutrition obeys the same laws as accidental nutrition, as that which takes place in the formation of fungous and sarcomatous tumours, and fleshy granulations. Now the cerebral nerves have evidently no connexion with all these productions; they are never found in them; a phenomenon very different from that which is offered by the arterial system, which is almost always developed in a remarkable manner in these tumours. In fine, we shall see hereafter that the nerves do not increase in proportion with the parts to which they are distributed.
From what has been said, it is evident that all the phenomena, over which preside what are commonly called the tonic forces, viz. organic sensibility and insensible contractility, are completely independent of nervous action; and consequently that these properties would not, like those of animal life, require this action. Each kind of sensibility has its morbid phenomena over which it presides. Inflammations, suppurations, the formation of tumours, dropsies, sweating, hemorrhages, disorders of secretions, &c. &c. belong to the alterations of the organic sensibility, whilst every thing like spasm, convulsion, paralysis, trance, torpor, injury of the intellectual functions, &c. &c. every thing, in fact, which tends in diseases to destroy our relations with surrounding bodies, belongs to the alterations of animal sensibility or contractility, and implies a greater or less degree of disorder in the nervous system.
In general, the diseases that affect the functions of animal life are of a nature wholly different from those which destroy the harmony of organic life. They have not the same character, the same progress, and the same phenomena. Place on one side the injuries of the external senses, blindness, deafness, loss of taste, &c.; those of the internal senses, mania, epilepsy, apoplexy, catalepsy, &c.; those of the voluntary motions, &c.; on the other, fevers, hemorrhages, catarrhs, &c. and all the diseases that disturb digestion, circulation, respiration, secretion, exhalation, absorption, nutrition, &c.; we shall then see what an immense difference there is between them.
Physicians have used too vaguely the term nervous influence. If in medicine, as in physiology, we had accustomed ourselves to use those expressions only to which was attached a precise and definite meaning, this would have been employed much less frequently.
It appears that the nerves have some influence still unknown, in the production of animal heat. The following facts relate to this influence. 1st. In aneurism, the ligature of the nerve is often followed by a sensation of general torpor and coldness in the limb. 2d. Sometimes, in hemiplegia, the affected part is of a temperature below what is natural, though the pulse may be as strong in this side as the other. 3d. One of the characters of ataxic fevers, the principal seat of which is in the brain, is the remarkable irregularity in the temperature of the different parts of the body. 4th. Animals, with a strongly marked nervous system, as quadrupeds and birds, are those of all others, in whom the degree of natural heat is the highest. 5th. I knew a person that had had the cubital nerve divided by a piece of glass above the pisiform bone, and whose little and ring fingers of that hand uniformly remained colder than the rest. 6th. Often in luxations, the compression of the nerves by the heads of the bones, produces an analogous effect, &c. &c.
However, the heat is not always increased, when the nervous action is augmented, nor is it always lessened when this action diminishes; there are as many cases in which the heat appears to be independent of the nervous system, as there are where it appears to be connected with it; so that we are still confined here to the collection of facts, without drawing general conclusions from them.
I divide what I have to say upon the sympathies of the nerves, as I did that which was said upon their vital forces; that is to say, I shall examine first the relations that each nerve has with the other parts, then I shall speak of the general influence that the nervous system exercises upon the sympathies and of the part it takes in them.
There is no doubt as to the relations of the nervous with the other systems, of those which it has with the muscles and with the brain. In fact, these relations are natural; for the one cannot be affected without the other's feeling it. These three organs can in this respect be considered under one point of view. Thus too, the pulsation of the arteries is always connected with the action of the heart, &c. Every idea of sympathy excludes that of a natural connexion of functions. Barthez is mistaken upon this point. I speak only of the unnatural relations of the phenomena that take place between an organ and a portion of the nervous system which is not connected with it by the natural order of life; now, thus considered, the nervous sympathies are very numerous.
1st. Two nerves of the same pair often sympathize with each other. We know in medicine the relation there is between the two optic nerves; the one being disordered in its functions, the other frequently becomes so. This happens more rarely with regard to the ears, the nostrils, &c. though it sometimes takes place in them. Often in neuralgia (a word which I adopt very willingly and which is wanted in the science to express a class of diseases every genus of which has a distinct name) often I say in neuralgia a nerve being painful, the corresponding one becomes so sympathetically. I have an example of this at present; it is a woman, who for two months has been afflicted with sciatica of the left limb. In changes of weather, a pain precisely similar spreads itself along the course of the nerve of the opposite side. I applied two blisters upon the thigh first affected; the pain disappeared in both sides at the same time at the end of twelve hours. Thus, in order to cure pains in both eyes, it is oftentimes sufficient to act only upon one, &c.
2d. Sometimes two nerves of the same side sympathize without belonging to the same trunk. Thus an injury of the frontal nerve has been many times followed by a sudden blindness in consequence of the affection of the optic nerve, &c.
3d. In other cases it is the branches of a common trunk which influence each other reciprocally, as when a branch of the superficial temporal is wounded in the operation of arteriotomy, the whole face, which also receives its nerves from the fifth pair, becomes painful, &c.
4th. At other times, it is not among themselves that the nerves sympathize, but with other organs; and then sometimes they influence, at others they are influenced.
I say first they influence; thus a nerve being irritated in any way, a number of sympathetic phenomena take place in the system. Diseases frequently show these facts. It is thus in tic douloureux and analogous diseases, in which the nervous texture is particularly affected, sometimes the animal sensibility is raised in various remote parts, and hence the pains that are often experienced in the head, in the internal viscera, pains that cease when the cause that supported them has disappeared. Sometimes it is the animal contractility; hence the convulsions that occasionally take place in the muscles that receive branches from the affected nerve. In some cases it is the sensible organic contractility which is sympathetically excited by the nervous affections. Thus in the paroxysm of neuralgic pains there is often spasmodic vomiting, the action of the heart is hurried, &c. We can by experiments produce the same phenomena. Thus by acting upon the nerves of the superior or inferior extremities, by irritating them any way after they have been laid bare, I have frequently produced vomiting, or convulsions in muscles that were in no way connected with the nerves I irritated.
In the second place, the nerves can be influenced by diseased organs; thus in many acute and chronic affections, sympathetic pains spread along the course of different nerves, particularly in the extremities. As the animal sensibility is the predominant property of the nerves, it is almost always that which is brought sympathetically into action. Physicians have not distinguished with sufficient accuracy in the pains of the extremities, that which belongs to the nerves, from that which has its seat in the muscles, the aponeuroses, the tendons, &c.
Authors have been much divided upon the cause that supports sympathies. How can an organ which has no relation with another that is frequently very remote, influence it so as to produce serious diseases there, merely because it is itself affected? This singular phenomenon is often witnessed in a state of health; but it is so wonderfully increased in diseases, that if we could remove from them the symptoms that are not exclusively dependant upon the derangement of the function particularly affected, we should find but little difficulty in their study or treatment. The moment an organ is affected, all the rest seem to feel simultaneously the disorder it experiences, and each seems to be agitated in its own way in order to expel the morbific matter that has seized upon one of them.
Most authors have believed that the nerves were the general means of communication that connected the organs with each other, and also their derangements. The anastomoses have appeared to them to be destined to this use; and with this opinion, some have thought that the brain was always mediately affected, but this others have rejected. The communication of parts by the means of blood vessels has also been thought to be a cause of sympathies. Others have admitted the continuity of the cellular texture; some, that of the mucous membranes. I shall not undertake to refute in detail these different hypotheses; I would observe only, that as no one is applicable to all the cases of sympathy, it is because the aberrations of the vital forces have been described in too general a manner; it has been thought that a single principle presided over them, and this principle has been sought for. But in order to ascertain the cause that supports sympathies, they must be divided, as I have divided the vital properties; for as each of these properties supposes different phenomena, so the sympathies that put them in action, differ also. To make this distinction of the sympathies more evident, let us suppose a diseased organ, the stomach for example; it becomes then a centre, whence go forth numerous sympathetic irradiations, which bring into action in the other parts, sometimes the animal sensibility, as when pains of the head come on; sometimes contractility of the same species, which is the case when worms in the stomach produce convulsions in children; sometimes, sensible organic contractility, which, raised in the heart by certain pains in the stomach, occasions fever; oftentimes the insensible organic contractility and the organic sensibility, as when the gastric affections increase sympathetically the secretions that take place upon the tongue, and produce there a mucous coat. There are then sympathies of animal sensibility and contractility, and of organic sensibility and contractility. This being premised, let us examine the cause of each.
1st. When the animal sensibility is sympathetically raised in a part, this does not always depend on nervous communications; for oftentimes the organ in which is the material cause of pain does not receive any nerves, as the tendons, the cartilages, &c.; then it cannot communicate by them with that in which this pain is found. On the other hand we have seen above that it is still very uncertain if the nerves are the only agent that carry to the brain the internal sensations; we cannot say then that the affected organ acts at first upon the brain by their means, and that this reacts afterwards upon the part in which the pain is seated, by the nerves that go to it. Can we conceive that the cellular texture should be an agent for the communication of pain, which is insensible to it itself? Observe also that the parts that are the most abundantly supplied with this texture, as the scrotum, the mediastinum, &c. are not those that sympathize the most. The same is true of the blood vessels, which, by their nature, are not fitted to transmit animal sensibility, and which besides do not exist in all the organs.
It appears that all sympathetic pains are nothing but an aberration of the internal sensitive principle, which refers to a part a sensation, the cause of which exists in another. Thus when the extremity of the stump gives the patient pain, who has just undergone amputation, the sensitive principle perceives the sensation correctly, though it is deceived as to the place whence it comes; it refers it to the foot which no longer exists. It is the same when a stone irritating the bladder, produces pain at the glans penis. Thus all sympathy of animal sensibility is characterized by the integrity of the part in which we find the pain, and by the cessation of this sympathetic pain, when the cause that acts elsewhere has ceased. It is then probable, when a part suffers sympathetically, that that which is the seat of the material cause of the pain acts first upon the brain, either by the nerves, or by some means with which we are unacquainted, and that when the brain perceives the sensation, it is mistaken as to it, and refers it to a part from which it does not arise; or it refers it at the same time to the part from which it arises, and to another where it does not; this happens frequently. The stone for example, produces suffering at the same time in the bladder and at the end of the glans penis.
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.
Are the nerves reproduced when they have been cut? The experiments of many distinguished anatomists evidently prove that they are. What is the manner of this reproduction? If we examine the results of these experiments it is easy to see that there is nothing peculiar in the nervous system, that it is a simple cicatrization analogous to the callus of bones, to the cicatrix of the skin, &c. When a nerve has been cut, its two ends inflame, the cellular texture that it contains sends forth granulations by the property of reproduction that it possesses. These granulations meeting, form adhesions that unite the two divided ends of the nerve. As the cellular texture, the means of union, grows from the cut extremity of the nervous coat, as well as from that which is between the cords, it partakes of the nature of the nervous coat, and becomes a parenchyma of nutrition, whose mode of organic sensibility is analogous to that of the nerves, and whose vessels deposit there medullary substance, which gives a new appearance to the nervous cicatrix, and makes it resemble very nearly the texture of the nerves themselves. However, as the granulations arising from the divided ends are not made in a regular manner, there is never at the place of union a thread-like arrangement as there is in the nerve itself. Thus the callus of a long bone, though analogous to this bone, is never regularly arranged like it in longitudinal fibres; thus a cutaneous cicatrix has always an irregularity in its organization, which arises from the irregular manner in which the parenchyma of cicatrization has been developed.
The cicatrization of nerves is then analogous to that of bones. In the first period there is inflammation; in the second, growth of the cellular texture which is to serve for the nutritive parenchyma; in the third, adhesion of those parts that have grown; in the fourth, exhalation of the medullary substance into the parenchyma. It is this medullary substance that makes this cicatrix differ from the osseous, in which phosphate of lime and gelatine are deposited, from the muscular, in which there is fibrin, &c. Sometimes there is an enlargement in the form of a ganglion, at the place of the reunion of the nerves; this depends upon the greater granulation of the cellular texture. Thus sometimes the callus is enlarged; at others, if the contact has been exact, we perceive but a slight difference; these are varieties that do not affect the nature of cicatrization.
It follows from this, that the regeneration of the nerves, which has lately been the object of much research, and which Cruikshank, Monro, &c. have particularly demonstrated, has nothing peculiar in it; that it is only a consequence of the general laws of cicatrization, and a proof of the constant uniformity of the operations of nature, though these operations present at first sight different results. A nerve, that is cut out in its whole course, is never reproduced like a nail, or the hair, which take a length, form, and appearance exactly the same as they had before they were removed. It is under the point of view that we have presented them, and not under this last, that the nervous reproductions should be described.
The nervous system of animal life is one of the first that is developed. If the heart is the first that has motion, the brain is the first that has any considerable size. The disproportion of the head to the other parts is remarkable in the first periods after conception; its size is monstrous when compared with that of the subsequent ages. Now it is evident, that it is the brain that produces this, and that the increase of the size of the bones and the membranes that surround it, is owing to it.
We may say that by creating first the heart and the brain, and developing them much sooner than the other organs, nature wished first to establish the foundations of the organization of the two lives. For on the one hand, it is the brain which is the centre of animal life; it is to this that all the sensations are referred; it is from it that all the voluntary motions proceed. On the other hand, by sending the blood towards all the organs, the heart evidently presides over the circulation, the secretions, exhalations, nutrition, &c. which compose by their union organic life. When these two essential bases exist, nature begins to build, or rather develop around them the double organized edifice, which produces on the one part a communication between the animal and external bodies, and on the other nourishes it.
Notwithstanding these early developments, the brain is not like the heart constantly active; its two great functions, relative to sensation and motion, are almost nothing. The intellectual functions also have but a very obscure action, if they have really commenced at all. The brain is then, if we may so say, in the expectation of action; it has not acted; it requires the excitement of external bodies. I do not say, however, that its inactivity is necessarily entire. It can undoubtedly perceive certain internal motions that take place in the body, and especially the pains that arise there; for if the organic diseases can produce the death of the fœtus, why does it not suffer pain in these diseases? Perhaps the brain is so much the more sensible to it, as it is not diverted by the external senses. The difference of the external and internal sensations, is a question that deserves to be attentively considered. We have seen that the first are uniformly transmitted by the nerves and that the mode of transmission of the second is uncertain. On the other hand the phenomena, the sensation, the impression, &c. are not the same in each; so that an examination of their relations and their differences is essential. This examination would have much influence upon the knowledge of the kind of animal life that the fœtus can enjoy. Whatever it may be, there can be no doubt but that it is infinitely more contracted than after birth.
The softness of the brain is very great in the fœtus; it is truly a kind of fluid, that the arteries, or rather the exhalants that arise from them deposit in their interstices. These arteries are then extremely numerous; as the brain has a very evident reddish tinge. When it is cut in slices, numerous streaks of this colour are observed in its substance. The two portions, the cortical and medullary, are infinitely less distinct than afterwards, because the second is much less white. The caustic alkali dissolves them at this period of life with great ease. The first effect before a complete solution, is to change the cerebral substance into a glutinous, transparent and viscous matter, a little reddish however, and ropy, almost like the white of an egg. I discovered nothing similar to this in my experiments with the brain of an adult when treated with caustic alkali. The acids coagulate the cerebral substance of the fœtus, it does not however attain by them a degree of hardness equal to what they produce in the subsequent periods of life.
The extreme softness of the brain renders its dissection very difficult in the fœtus.
The nerves of animal life have a development proportional to that of the brain. All of them are very large compared to the other parts; thus the fœtus and the young infant are the most proper for the study of the nervous system, as the less development of the other systems renders this more apparent. Their medullary substance is, like the cerebral and that of the spinal marrow, very soft and even almost liquid under the finger; in this state we can see it in the anterior part of the optic, in which it is very evident though contained in the canals of the nervous coat, in the posterior part of the same nerve, and in the olfactory where it is found by itself, in the auditory in which it predominates, and finally at the origin of each pair, where its proportion to the nervous coat is very evident.
In all the other nerves it is much more difficult to examine well this medullary substance, because the nervous coat that contains it, is as much or even more developed in proportion to what it will be afterwards. Hence it is that the nerves are very hard and resisting in the fœtus; and that they can support weights proportionably very great. Maceration in water, at a moderate temperature, increases this resistance as in the adult, and renders the nerve harder without increasing its size. We should say that this fluid acts at first upon the nervous coat, in an opposite manner to what it does upon the other animal substances; finally it softens it also, and renders it almost liquid.
The blood vessels are proportionably much larger in the nerves of the fœtus than in those of the adult. These nerves have in their whitish colour, a livid tinge that arises from the kind of blood that enters them; it is the same phenomenon as that of the brain.
The development of the cerebral nerves in the first age presents a phenomenon which essentially distinguishes it from the development of the arteries. These last always follow the increase of the parts to which they go. Thus, the face proportionably less developed in the fœtus, has less large arteries. It is the same of the viscera of the pelvis, whose very small arteries receive but little blood, which does not penetrate and dilate them until the umbilical are closed. On the contrary, the size of the cerebral, gastric arteries, &c. is very considerable. The nerves are absolutely independent in their increase, of that of the parts to which they are distributed. The olfactory, whose organ is so contracted in the fœtus, has the same proportion as the optic and the auditory, whose organs are already so much developed. It is the same of all the nerves of the voluntary muscles; their proportion of development is uniform, though the muscles vary in their size, according to the regions. If without regard to these regions, we examine in a general and comparative manner the nervous, cerebral and animal muscular systems, we shall see that the first then predominates manifestly over the second, while in the adult it is the muscles, which proportionably to what they were in the fœtus, surpass the nerves that are sent to them. The par vagum which is distributed to organs whose increase is not in the same relation, presents nevertheless the same proportion of size as afterwards, in its different branches.
This double opposite arrangement of the two systems the arterial and nervous cerebral, proves on the one part, the immediate relation of the first with the increase and nutrition, and on the other the small influence that the second exercises upon them.
The nerves are, like the brain, principally inactive before birth, though they have a great development. It is to this that must be attributed the constant absence of their affections at this period.
The nerves are always found in the fœtus, whereas the brain, and even the spinal marrow are sometimes wanting; this is what constitutes acephalic subjects. I shall say elsewhere how the fœtus can thus exist. I would only remark here that the heart, the liver and the other principal viscera of organic life, are on the contrary rarely deficient in the fœtus. Why? Because all the essential organs of this life are necessary, for growth, vegetation and nourishment, phenomena that can take place without the cerebral influence which is principally destined to preside over animal life, which is not particularly in exercise until birth.
At birth the animal nervous system experiences a remarkable revolution, in consequence of the red blood that penetrates it. Heretofore black blood only circulated in its vessels. The sudden difference that the circulation experiences, has a manifest influence upon its functions. In fact the least foreign substance, differing from red blood, which during life is forced towards the brain by the carotid, is sufficient to produce there a remarkable derangement, and oftentimes even death, as I have frequently convinced myself. Why? because it is not only as a vehicle of nutritive matter, that the fluid sent by the arteries acts upon the brain, but also as an excitant, a stimulant. The change of excitement which the brain suddenly experiences at birth, inevitably increases its vital activity, gives it that which is new and renders it fit for the functions it has never before performed, those of receiving sensations.
Asphyxia is real always when the lungs are not developed after birth, when they do not receive air and consequently do not send red blood to the brain. Some muscular motions may undoubtedly be made; but animal life never begins in its perfection, until the organs that execute it are influenced by red blood. This blood is a general cause of internal excitement. This direct acts simultaneously with the sympathetic excitement that the brain experiences from the skin and mucous surfaces, which the external agents act upon immediately after the exit of the fœtus from the womb. The lungs and the brain influence each other reciprocally at this period, the first by sending red blood to the second, and this by putting in action the diaphragm and the intercostals, which make the air, that is necessary for the production of this red blood penetrate the lungs; hence we see that other excitants act before that of this blood, since before its formation, the brain has already in it a principle of motion.
Besides, the brain and the whole nervous system are the more powerfully excited by the new principles that the blood has derived from the air, as, 1st. their vessels are in proportion larger and more numerous than afterwards; and, 2d. as all the cerebral arteries enter at that part of the base of the brain, where is found the origin of the nerves, and which is without doubt the most sensitive part of the whole organ.
There is certainly a very great difference between asphyxia that happens to an adult, and the state in which the fœtus is found, since, if the first is prolonged, organic life ceases, while this life is in full activity in the fœtus. Thus there is no resemblance in the composition of the black blood in the arteries in asphyxia and that in the arteries of the fœtus. These two states, however, have a sort of analogy, especially under the relation of the remarkable diminution, of even the absence of animal life, which characterize both. Now in producing asphyxia in an animal at will, by fixing a stop-cock upon the wind-pipe, I have always observed that this life is annihilated when the black blood penetrates the brain, and that when it is in part suspended, it suddenly revives and re-appears by opening the stop-cock, and permitting red blood to enter the brain, nerves, and all the parts. These experiments can, then, to a certain point, give us an idea of the part the red blood takes at birth, in the development of animal life; I say the part, for it is not, as we shall see, the only cause that puts it in action.
For a long time after birth and during the whole of the growth, the nervous system and the brain, which is the centre of it, predominate in their development over the other systems; this predominance is not uniform at all the periods; it diminishes at puberty, when the nervous system is in equilibrium with the others, and the genital organs succeed it in superiority.
This predominance of the nervous system in the infant has an influence on the one hand on the sensations, on the other upon the voluntary motions.
The first influence is very striking. Infancy is the age of sensations. As every thing is new to the infant, every thing attracts its eyes, ears, nostrils, &c. That which to us is an object of indifference, is to it a source of pleasures. As a man receives great enjoyment from a show he never witnessed before, which is blunted by habit if often repeated. It was then necessary that the nervous cerebral system should be adapted, by its early development, to the great degree of action which it is then to have. In fact, all the organs that receive external impressions, the nerves that transmit them, and the brain that perceives them, are really in the infant when awake in permanent excitement, who in the midst of the same objects as the adult, fatigues these organs three times as much as he to whom a great part of these external objects is indifferent, because they have heretofore excited him. Thus observe that the periods of activity of animal life are much shorter in the infant who fatigues his organs in a few hours, in whom, consequently, the want of sleep returns oftener, and in whom this state of intermission of animal life is more profound. It is rare that infants, in the first months, can pass the whole day awake, especially if many objects engage them. We might prolong their wakefulness by removing them from light, sounds, &c.
The multiplicity and frequency of the sensations of the infant, lead necessarily to a number of motions which have not strength, because of the weakness of the muscles, but which are, like the sensations, extremely numerous. As the sight incessantly presents new objects to the infant, it wishes constantly to touch; its little hands are in continual agitation, its whole body is also in constant motion. It is necessary that the nerves which serve to transmit the principle of these motions, should be adapted by their development, like those of the sensations, to their constant action.
These two things, the great development of the nervous system and the frequency of its action in the infant, make the diseases of this system the predominant ones at that age. So great is the susceptibility of the brain in answering to sympathetic excitements, that if pains are at all severe in any part, they immediately produce convulsions, which are at least four times more frequent at this age than any of the following. I would observe upon this subject, that the different systems are more or less disposed in the different ages, to answer to sympathies, according as their predominance in the economy is more or less decided. The same morbific cause, fixed upon any organ, which produces convulsions in an infant by acting sympathetically upon the brain, would give to a young girl a suppression of the catamenia, by influencing the womb, which then begins to predominate; to a strong vigorous young man, a peripneumony; to an adult, in whom the gastric viscera predominate, an affection of these viscera, &c. It is thus that the same passions that would give to this one a jaundice, engorgement of the liver, &c. would produce more particularly in an infant an epilepsy, which attacks the brain.
The nervous functions are not only frequently deranged by sympathy in infancy, but it is particularly at this age that the greatest number of organic diseases is found in the brain, the spinal marrow, the nerves, or the organs that depend upon them. Cerebral fungi, hydrocephalus, spina bifida, &c. are a proof of this. The great quantity of blood that goes at that period to the nervous system has much influence upon this phenomenon; now this quantity is brought there by the predominance of the vital forces.
In proportion as the infant grows, its nervous system and the brain, which is the centre of it, lose by degrees the predominance that characterize them. Their diseases become less frequent. They are brought finally to the level of the other systems.
At puberty, the empire of the brain, which is insensibly diminished, gives place to that of the genital organs, which have a sudden increase. The cerebral nerves appear to me to have but little influence upon their development, as well as upon that of most of the other systems. Observe, in fact, that all the phenomena of generation are governed by the organic forces, which, as we have seen, are absolutely independent of the nerves. Thus the great excitement of the genital organs, from which arise satyriasis, nymphomania, &c. have no analogy with convulsions whose principle is in the brain; as the destruction of the venereal appetite is wholly disconnected with the phenomena of palsies. This is so true, that often during those that affect the lower half of the body by a fall on the sacrum, or by any other cause, the secretion of semen and venereal desires take place as usual.
Beyond puberty, and towards the adult age, when the general equilibrium is more nearly established among the different systems, the nervous is not affected more than those of which we have had occasion to speak in treating of this system.
At this period of life, the nervous cerebral system has but very few functions to perform. As to sensation, this being almost blunted by habit, is the reason why external bodies make but little impression upon the organs of sense; many of these, especially the eye and the ear, are often shut to sensations before general death. The nerves have then but little to transmit, and the brain but little to perceive. As to motion, there is but little in old age, because but little is felt; for feeling and motion are two things that generally follow the same proportion. The brain and the nerves are almost inactive in this respect. The first is not put in action by the intellectual functions; memory, imagination, judgment, attention, &c. all are enfeebled, none are exerted with clearness.
Changes of structure constantly accord with these changes of functions. In the fœtus, the brain is almost fluid; in old age, it is extremely firm. This organ has passed through a variety of gradations between the two extreme ages. We know that anatomists always select the brain of an old person in order to study this viscus, all the parts of which are broken with difficulty. I would observe upon this subject, that what is natural at this age, indicates in a young person a morbid alteration. In general, we have not yet sufficiently studied the comparative anatomy of the different ages, to make applications of it to the examination of dead bodies.
The vessels diminish in the brain in proportion as its hardness increases. In this respect it has an inverse arrangement at the two extreme ages of life. Its colour becomes more dull in old age. It is rare that it is ossified; there are, however, some examples of it. The phenomena, that the action of different re-agents presents, are very much slower in taking place than in the adult and especially in the infant. The solution by alkalies is a remarkable proof of this.
We cannot doubt but that this organic state of the brain in old age, has much influence upon the preceding phenomena; to this must be referred the less acuteness of pain at this age. A cancerous tumour of an old person, exactly analogous in its position, form, size and nature to that of an adult, produces much less suffering. Cancers of the womb, the stomach, the breast, &c. furnish examples of this. All the local causes of pain show also the same thing. In the numerous experiments I have made upon living animals, I have uniformly observed, that young ones, when the sensible parts are cut, give signs of the most acute pain; whilst old ones show infinitely less expression of it under similar circumstances. I would make one other remark upon this subject; it is that the variety appears in dogs, in a certain degree to have an influence upon the acuteness of their sensations. All the large varieties make but little noise, and are not much agitated, when their skin, their nerves, &c. are cut; whilst all the small ones, though they may be old, struggle, are agitated, and manifest upon the slightest cause, the most acute sensibility.
As to the influence of age upon pain, it is not astonishing that the animal sensibility having become very obscure in a natural state, should preserve the same character in disease. An old person suffers then much less than the adult, and especially than the infant, under the influence of the same causes; it is a compensation for the diminution of their enjoyments. The infant finds in every thing that surrounds him, a cause of pleasure or of pain; thus smiles and tears succeed each other a hundred times a day upon his little face. An old person on the contrary is always calm; indifference is his natural state.