Part 17

This property would seem at first to establish a very great difference between the medullary substance of the nerves and that of the brain, especially towards the convexity of this organ; for we can almost with impunity irritate this after having removed the cortical substance. It is only deep in the brain that the animal sensibility becomes strongly marked, and even there it is not so much so as in the nerves. Observe, however, that in the experiments upon the cerebral pulp you destroy the organ itself that perceives, that, without which it could not have animal sensibility, and whose derangement would consequently have an inevitable influence upon this property; whereas the seat of perception being untouched when we irritate a nerve, the pain is more sensibly felt. It is, in fact, principally in the medullary substance of each nervous filament, that animal sensibility resides. The nervous coat possesses a much less degree of it. Hence why simple contact, without pressure, occasions but little pain; hence, also, why a nerve can almost with impunity be immersed in a purulent, ichorous fluid, or even in the sanies of cancer; hence why the contact of the air occasions but little pain when the nerves are merely laid bare, as I have had frequent occasion to see in animals; why, in a variety of cases, different tumours, in whose atmosphere the nerves are situated, have no influence upon them. The membrane of each filament is truly in every case a kind of covering that protects its medullary substance, in which the sensibility particularly resides. As to the cellular texture which enters into the composition of nerves, it has, as elsewhere, no connexion with this property. Hence why we can, as I have often done upon a living animal, separate from each other, with the point of a very delicate scalpel, the different filaments of a nerve of some size, of the sciatic, for example, when they have first been laid bare, without giving the animal much pain. In these experiments, it is easy to be convinced of the kind of insensibility of the covering of each nervous filament. It is necessary to pierce it and arrive at the medullary substance in order to produce pain.

In experiments, the animal sensibility of the nerve seems to be gradually exhausted, and finally ceases. I convinced myself of this upon the eighth pair of nerves, in making my experiments upon the injection of black blood into the brain. At the moment the nerve is raised and drawn to detach it from the carotid with which it is connected, the animal cries out and is much agitated; but after the same thing is repeated two or three times, he no longer gives any signs of pain. If we cease to excite the nerve for an hour or two, the sensibility returns with great energy, when it is drawn again. These experiments furnish a result very analogous to that of experiments relative to the animal contractility of muscles, experiments that are known to all physiologists.

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.

_Influence of the nerves upon the animal sensibility of all the organs._

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?

_Animal contractility. Influence of the nerves upon that of the other parts._

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.

_Properties of organic life, considered in the nerves._

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.