Part 16

3d. The medullary substance of the nerves, as well as that of the brain and spinal marrow, does not seem to be susceptible of any kind of horny hardening. This is very evident when we immerse the two last in boiling water, in a concentrated acid, &c. We may be convinced as to the first, by submitting to the same experiment the soft nerves that have their nervous coat pretty distinct. To this also must be referred the following phenomenon; when the anterior part of the optic nerve is put into boiling water, the nervous coat becomes wrinkled, its canals shrink, and the medullary substance not contracting in proportion, is forced towards the extremities, which become consequently enlarged. As this substance is in less proportion in the other nerves, this phenomenon is less apparent in them; it takes place, however, and this explains the small round tubercle that is seen at each end of boiled nervous filaments; it is the medullary substance that produces these enlargements. This phenomenon is very evident in the spinal marrow, which, being immersed in boiling water, suffers the compressed substance to escape, either at the extremities, or at any openings that may be made in its covering. Thus in boiling a head, the dura mater detached from the cranium contracts powerfully in hardening like horn, compresses the cerebral substance which does not contract like it, and sometimes breaks it, so that it escapes into the space that the boiling has produced between the dura mater and the cranium.

4th. When the cerebral substance is agitated in water, it becomes suspended in it in the form of an emulsion, as has been observed by Fourcroy, then it is precipitated to the bottom of the vessel. A similar emulsion is made by the olfactory nerves, the posterior part of the optics, &c. When the anterior part of these, in which the nervous coat is very evident, has been soaked some time in water, and commonly even without this, a large quantity of whitish substance can be pressed out of them, which is evidently analogous to the medulla of the brain, and which colours the water that receives it. From the other nerves in which the medullary substance is much less abundant, it can often be forced out by pressure, from the cut ends of the filaments, especially if they have been previously macerated in an alkaline solution.

5th. Boiling hardens the brain, and gives it a greyish and dingy appearance, very similar to what is seen in ataxic fevers. The same phenomenon takes place in the soft nerves. In the others, the nervous coat is in too great a proportion to the medullary substance to allow us to see what happens to this last. It is to this property of coagulating by heat, which the brain has, that must be referred the flaky precipitate that is obtained in a heated cerebral emulsion.

6th. All the acids that are much concentrated harden the brain very evidently, the instant it is immersed in them. The sulphuric afterwards softens it, and finally reduces it to pulp, if it is not diluted. The nitric makes it yellow only, in hardening it. The muriatic has the least action upon it. The effect of acids upon the soft nerves is very analogous to this. In those in which the coat is very distinct, the horny hardening of which this coat is the seat, conceals all the sudden phenomena relative to the medullary substance. When the coat is softened and dissolved, this substance has appeared to me to be diminished in consistence and altered by the acids, whereas that of the brain keeps always the same degree of hardness, if the acid be not too much concentrated.

We all know that alkohol hardens the brain. This hardening, the effect of acids, of boiling, and of alkohol, is a phenomenon that the anatomist can avail himself of to give the parts he dissects a firmness, that will enable him to examine them better. It approximates this substance to the albuminous fluids. I say that it approximates it, for there are still great differences between them, of which, I think, we know but little.

7th. The alkalies have an effect upon the cerebral substance precisely opposite to that of the acids. They make it fluid, and even dissolve it completely after a short time. I have observed, with regard to this, that the grey substance is much quicker altered by them than the white, which is softened, disappears in part, but still leaves a considerable portion that is not dissolved. From whatever part we take these two substances to submit them to the action of the alkalies, the result is the same. The alkalies act also evidently upon the medullary substance of the nerves. This action, as I have said, has been of great assistance to Reil in his experiments.

8th. Thouret and Fourcroy have discovered, that the brain, after being buried, lessens considerably in size, and changes to a brittle substance, capable of softening under the finger, miscible in water, exhaling a disagreeable odour, having the properties of ammoniacal soap, and resembling very closely spermaceti in its nature. Do the nerves undergo a similar alteration in their medullary substance? We know nothing at present by which we can determine this question.

9th. The muriate of soda, when sprinkled upon slices of the brain and the pulpy nerves, increases their consistence.

10th. The digestive juices generally alter with ease the medullary substance of the brain. I think, however, that they would have a much more powerful action upon it in a natural than a boiled state; for all the re-agents are in general more powerful in the first of these states. We know that most carnivorous animals esteem the cerebral substance delicious food. Those that feed upon birds, the parietes of whose craniums are easily broken, almost always eat the brain first. The weasel, the polecat, &c. furnish examples of this. Man considers the brain as one of the most dainty parts of the animal. The nerves are much less easily digested; but this depends wholly upon their coat, which does not yield so readily to boiling as the other parts. For example, the tendons, which are as hard or harder than the nerves in a natural state, become much softer by boiling. We can distinguish in boiled meat each of these parts. The first, in its gelatinous state, is more pleasant and digestible.

11th. The cerebral medullary substance is very different in the brain, the tuber annulare and its elongations, and the spinal marrow. If we examine it attentively, in all these we must perceive the difference in colour, consistence, hardness, humidity, and, without doubt also in its very nature, though our knowledge is not yet sufficiently advanced to decide with certainty upon this last point. Has the nervous medullary substance analogous differences? I believe that it is similar in the same nerve, but that it varies in different nerves according to their uses. In fact, when the internal arrangement of the cords and the filaments which constitute the nerve, differs so much, when there are varieties in the nervous coat also, why should the medullary substance be every where of the same nature? Certainly the colour and consistence of that of the olfactory are different from that which is forced out from the anterior part of the optic. That of the auditory does not resemble that of the trigemini, &c. We have seen that each of the organs of sense has its peculiar sensibility, which places it exclusively in relation with particular bodies in nature, that of the eye with light, that of the ear with sounds, &c. I think that these differences of sensibility depend upon the difference of organs; but I am persuaded that the organization of the nerves has much influence, and that the optic nerve would be unfit to transmit tastes, the auditory to propagate impressions made by light, &c. If we examine attentively, we shall see an essential difference of structure between the nerve of the eye, of the nostrils, the ear, and that of the taste, which approximates, in thickness, the nerves of motion. As to the nerves of touch, they do not require a peculiar texture; for I shall prove hereafter that a particular kind of animal sensibility is not necessary for this sense, but that this general property is sufficient for it, since its accuracy depends especially upon the mechanical form of the hand. As to the nerves that go to the voluntary muscles, as these muscles are every where analogous and perform similar functions, I think their medullary substance is the same. But in the par vagum, whose destination is so different, why do not the varieties of internal organization coincide with that of the texture which we observe in dissecting this nerve? We may say the same of many nerves that go to parts whose sensibility presents an entirely different modification.

This then is a comparison between the cerebral pulp and the medullary substance of the nerves, which may throw some light upon their difference and their analogy. I have not availed myself of all the details of the chemical experiments that have heretofore been made upon the brain; I have only given the principal phenomena of the action of different re-agents, phenomena, all of which I have repeatedly proved.

The medullary substance of the nerves is not arranged in filaments. It appears to be analogous to the white substance of the spinal marrow which is a real jelly, stagnant in the canal of the pia mater, which serves as a reservoir for it. Besides, examination proves this assertion in the optic, auditory, olfactory nerves, &c. In general I think, that this substance, as well as the cerebral, would be ranked, if they were deprived of the vessels that run through them, rather among the fluids than the solids, or they would form a medium of connexion between the two.

II. _Parts common to the organization of the nervous system of animal life._

_Cellular texture._

The nerves are entirely destitute of this texture in the interior of the cranium, and the spine; but out of these they have a great quantity of it. A large external layer, first covers and then connects them with the neighbouring parts. This layer is looser than that which surrounds the arteries. Fat often accumulates in it; sometimes, though rarely it is the seat of dropsical effusions.

From this common layer go off different elongations which communicate with the cellular texture of the neighbouring organs, and form a medium of union between the nerve and these organs. Within, there are other elongations that go between the nervous cords, and separate them from each other, and form for them kind of canals. When a nerve has been macerated some time in diluted nitric acid, the cords become separated from their sheath, which is to them what the layer of which we have spoken is to the whole nerve. These cellular canals often contain also fat in the great nerves, in the sciatic there is always some of it. Hence it is that when we dry these organs, there is almost always as I have observed, a fatty exhalation upon their surface; and that, when they are immersed in any alkaline solution, they have evidently an unctuous and truly saponaceous deposit.

Finally new elongations going from the cellular canals that surround the cords, cover the nervous filaments with canals still smaller. Here, there is never any fat or serum, and the cellular texture has in part that peculiar nature which characterizes the sub-arterial, the sub-nervous texture, &c.; perhaps even the nervous coat is nothing but this texture considerably condensed. Besides, the cellular texture so connects the one to the other, the cords of the nerves, and the filaments of these cords, that no motion can take place there.

_Blood vessels._

Each nerve receives its vessels from the surrounding trunks, which send to it branches which penetrate from all sides to the interior. The optic is an exception to this rule; the membrane that surrounds it prevents the vessels from entering it laterally. An artery passes through it in the course of its axis and sends out different branches.

In the other nerves, the arteries run first in the cellular texture between the cords, and are of a size there more or less considerable, according to the nervous trunks. Sometimes this size increases considerably. For example, in popliteal aneurism, the artery of the sciatic nerve has been seen with a caliber more than three times its natural size.

The arteries running between the cords, send off a number of little branches that go into all the interstices of the filaments. In fine, from these arise the little capillary arteries which are spread upon the coat of the nerve, cross it and are continued with the exhalants of the medullary substance. We readily see this vascular arrangement in the spinal marrow. Numerous ramifications are spread first upon the dense and firm pia mater, which there takes the place of the nervous coat; they then penetrate the medullary substance, and are lost in continuation with the exhalants.

The veins follow in the nerves a course analogous to the arteries; however, in carefully dissecting many great nervous trunks, I have been convinced that their branches do not go out of the nerves at the place where the arteries enter. This arrangement is analogous to that of the brain, where the arteries enter below and the veins go out above.

Many authors, particularly Reil, have overrated the quantity of blood that goes to the nerves, because, in order to ascertain it, they have made use of fine injections which have entered the capillary system, which does not commonly contain red blood. I am convinced by dissecting the nerves of living animals, the only means of having an accurate idea of what takes place in a natural state, how uncertain this method is here as every where else.

The blood that goes to the nerves, like that which is sent to the brain, is a stimulant that supports their action. When this stimulant is increased the nervous excitability increases, as Reil was convinced by rubbing the nerves of a frog, so as to redden them by the quantity of blood that was brought there. Does this fluid, when carried in great quantity to the nervous system, sometimes interrupt its functions, as happens to the brain in sanguineous apoplexy? I have not had an opportunity yet of making this observation in a very decided manner in the great number of bodies that I have opened. Only the nerves are a little more reddish in some cases than others. Do these cases coincide with certain determinate diseases? I have not any knowledge upon this point. As to the pretended compression of the origin of the nerves, by the blood that is carried to the brain and spinal marrow, whoever has examined the relations of the nerves with the vessels of the base of the cranium, will see that such a compression is not probable. Besides, most of the holes through which the little arteries penetrate into the interior even of this viscus, have a caliber greater than that of the arteries; so that how full soever they may be, they cannot press upon their parietes. I can only conceive of a compression at the origin of the nerves, from effusions at the base of the cranium.

_Exhalants and absorbents._

We cannot discover these vessels in the nerves; but nutrition supposes their existence there. It appears that this function is performed in the following manner; the exhalants receive from the arteries, with which they are continuous, the medullary substance which they deposit in the canal of the nervous coat, which is, if I may so express myself the reservoir of this substance, that is afterwards taken up by the absorbents.

Many think that the nervous coat is the secretory organ of this medullary substance, which oozes out of its parietes, to lie in its cavity. I do not believe it, 1st. because the olfactory nerve would not then be able to support itself, any more than the posterior portion of the optic. 2d. The cerebral membranes are not concerned with the secretion of the pulp of the brain, they only permit the vessels to pass, which enter this organ to deposit it there. 3d. There is the same arrangement at the spinal marrow, the pia mater of which has so great an analogy with the nervous coat. The vessels cross this membrane, then losing themselves, as I have said, in the medullary substance constantly renew it; so that if it was possible to remove this substance without touching the vessels, these would hang loose by their extremities in the canal of the pia mater. Thus, in some very soft fungi, the vessels cross here and there the substance that they deposit in their interstices, and would produce a vegetation like net-work, if we could remove this substance and leave them untouched. 4th. In the optic nerve the vessels evidently are not confined to the nervous coat; they penetrate also the canals it forms, and deposit there the medullary substance.

Every thing appears then to prove, that the nervous coat is no more the secretory organ of the nervous substance, than the pia mater is of the cerebral substances or that of the spinal marrow. It may have uses of which we are ignorant; but the principal certainly is that of a covering; it is the passive part of the nerve, the medulla being the part essentially active.

From this way of describing the production of the nervous medullary substance, it is evident that it does not proceed from the brain, but that it is formed in each nerve by the means of the neighbouring vessels. Hence why the inferior portion of a cut nerve does not decay; why a ligature that interrupts the cerebral communications, does not prevent the nervous nutrition; why in most paralyses in which the nervous system ceases to correspond with this organ, it is supported as usual.

From these and other considerations, Reil considers the nerves as having an entirely insulated existence, as being bodies by themselves, communicating only on one side with the brain, on the other with the different parts. This assertion is true as it respects nutrition, but as it regards the functions it is in part false; for the nerves are evidently only conductors; it is from the brain that goes the impulse, and there too is the sensation. In animals with white blood, and even in those with red and cold blood, these functions concentrated in the brain, in man and the neighbouring species, are, it is true more generally spread throughout the nervous system, hence it is without doubt, that we can remove the brain, the heart and the lungs in reptiles without immediately destroying life; it is on this account, that I have remarked in my Researches upon Death, that we should never avail ourselves of experiments upon animals with red and cold blood, to draw conclusions concerning those with red and warm blood. But in these and in man especially, it is undeniable, 1st. that the brain is the centre of animal life, which ceases when the action of this viscus is destroyed, as is proved by apoplexy, asphyxia, &c.; 2d. that it has also immediately dependant upon it organic life, though in an indirect way, that is by presiding over the mechanical functions of respiration, which by ceasing, stop the chemical, then the circulation, then the secretions, &c. so that the continuance of the two lives, and a serious injury of the brain, are two things wholly incompatible. Authors who have written upon life, the nervous system, &c. have usually considered them in too general a manner. The relations of the functions are absolutely different in animals with cold blood and in those with warm; that which is true for one, is not so for the other.

_Nerves._

Does the nervous coat receive small nervous branches? Do these small branches penetrate the nerves, as the small arteries spread on the coats of the large ones? Anatomical examination does not render this probable.

ARTICLE THIRD.

PROPERTIES OF THE NERVOUS SYSTEM OF ANIMAL LIFE.

I. _Properties of texture._

Few systems exhibit these properties more obscurely than this. If we draw a nerve, in an opposite direction, in a living animal, it is extended with difficulty, makes great resistance, and acquires a length but little more than what is natural to it; this appears to depend particularly on the nervous coat. The medullary substance would yield much more. We know how much that of the brain is stretched in the dropsy of the ventricles. If a great trunk is distended by a subjacent tumour, as in popliteal aneurism, by a swelling in the axilla, &c. it is flattened down like a ribbon; its filaments are separated and lay at the side of each other, and it is consequently much widened. Thus distended, these filaments can yet sometimes transmit sensation and motion, at other times these two functions are annihilated there.

In general, a sudden distension interrupts them much more certainly than that which comes on slowly. Hence why the luxation of the head of the humerus often occasions paralysis, whilst it rarely happens from very large chronic tumours in the axilla. Spontaneous luxations of the vertebræ, which always come on slowly, are rarely accompanied by paralysis, an accident which is always the result of those that happen from external violence. It is thus in the brain, osseous tumours, large fungi which increase slowly, disturb its functions but little, while the least depression of a bone of the cranium, that succeeds a fracture, entirely deranges them. In hydrocephalus, also, a great collection of serum has oftentimes but little effect upon sensation, which is nearly destroyed, when a little more of this fluid than common is exhaled in the ventricles, as happens in some kinds of apoplexy.

When a large cavity, like the abdomen, is distended, the nerves that are there yield partly because their curves disappear, and partly because they are really elongated; there is also a greater separation of them.

The contractility of texture is still less evident than the extensibility. A nerve cut transversely, does not retract at the two ends, which remain opposite to each other, like those of a tendon. In amputation, the end of the nerve remains longer than those of the muscles, the skin, &c. This is sometimes the cause of a painful pressure from some part of the dressing.

II. _Vital properties._

These are less evident in the nerves, than would be thought at first, from the opinions of a great many physicians, who have made these organs perform almost the whole part in diseases.

_Properties of animal life._

The nerves must be considered, in regard to sensibility, in two points of view. 1st. We should examine that which is inherent in them. 2d. It is necessary to consider the part which they take in that of all the other organs.

_Animal sensibility inherent in the nerves._

This property is, of all others, the most strongly marked in the nerves. Exposed and irritated, they cause great pain. By tying, pricking, cauterizing, or exciting in any way a nervous filament, we uniformly obtain that result so well known in practical surgery, and by those who make experiments upon living animals.