Chapter X: Section II (5)

The origin of the nervous coat is very evident at the spinal marrow. It is continued with the dense and compact membrane which covers its white substance, and which is called the pia mater, though it does not resemble the membrane of that name which surrounds the cerebral circumvolutions. To see this origin well, this spinal membrane should be cut longitudinally before or behind. The medulla then appears whitish, soft and easily raised up. If it is raised and scraped with a scalpel or any other instrument, the immediate covering of the spinal marrow is thus separated from either side, especially if precaution be taken to wash it. It might be had in the form of a sac, by cutting out a piece of the medulla of a certain extent and then pressing out the medullary substance at the two ends. In this double experiment, the nerves remain attached to the membrane separated from its medullary substance, because their nervous coat is continued with it. It is exactly as if a number of small arterial filaments went from the aorta; the parietes of this artery would be to those of these filaments, what the pia mater of the spinal marrow is to the coat of the nerves which go from it. Only the nerves are white, because their medulla fills them; whereas the canal to which they belong is transparent, because it is deprived of its own medulla. I do not pretend however, that there is a perfect identity between these two membranes, since we do not exactly know the nature of either; I refer only to their anatomical arrangement.

As to the origin of the nerves contained in the cranium, those coming from the tuber-annulare and its dependancies, that is to say, the elongations that it receives from the cerebrum and cerebellum, have an arrangement analogous to that of the nerves of the spine. However, the difference of thickness and density of the pia mater establishes differences. In fact the pia mater which covers these parts is different from that which serves as a canal to the spinal marrow; it is much softer, less adherent, is torn with more ease, and appears to be analogous to that which covers the cortical substance of the brain. The coat of the nerves of the tuber annulare, which is manifestly continued from this portion of the pia mater, exhibits partly this character. At the place of their union, it is more soft than in the canal, hence the extreme facility with which, as I have observed, the origin of these nerves is broken. Moreover, the continuity with the pia mater is proved by the facility of raising the nerves by raising this membrane; almost always both are attached together.

As to the nerves of the cerebrum, the olfactory, loosely covered by the pia mater, does not appear to have a coat of its own. The optic is evidently destitute of it from its origin to its junction with that of the opposite side. Then it begins to be covered with it; and canals are formed by it, filled with medullary substance, and which continue even to the retina. Besides, this nerve differs singularly from the others, 1st. because it has a kind of general nervous coat; 2d. because its medullary substance is more abundant and more easily obtained, its canals being larger; 3d. because these canals, pressed against each other, give it the appearance in the interior of a continued body; but by cutting it longitudinally, it is easy to see that the medullary substance is separated there by partitions. The auditory nerve has also a very peculiar texture.

From what has been said, it is evident that the pia mater has greater analogy with the coat of the nerves, than any of the other membranes; it may be said to be almost the same in the spinal canal. Observe, in fine, that this membrane, which has never yet been well described, evidently presents three great modifications, according as it is examined; 1st. upon the grey substance that surrounds the whole of the cerebrum and cerebellum, where it ¡s reddish, extremely vascular, loose, slightly resisting, and very easily raised; 2d. upon the white substance that covers anteriorly and posteriorly the tuber annulare and the four great elongations that it receives from the cerebrum and the cerebellum, where it is less red and where it begins to become more firm, more adherent, and less easily torn; 3d. upon the whole spinal marrow and upon the corpora pyramidalia and olivaria. It is thickened and condensed at the furrow that separates these eminences from the tuber annulare, then, increasing in thickness below, becoming whitish, resisting, &c. it has an appearance entirely different from what it had in the cranium. It might be said to be a membrane wholly different. It has four times the thickness of the tunica arachnoides.

In most of the subjects that I have examined it is much stretched, and compresses, if it may be so said, the medullary substance for which it serves as a canal; so that when a small opening is made in it the medullary substance immediately comes out. But I presume that it is looser during life. Besides, this state of compression is much less sensible towards the superior part than towards the middle and inferior, on account of the difference of thickness. I would remark, that the density of the pia mater of the spine is necessary to prevent injuries of the medullary substance, which is very soft at one part, and which at another is smaller than the diameter of the canal; so that it can be shaken there; an arrangement wholly different from that of the brain, which completely fills the cranium.

Arising in the manner we have pointed out, the coat of the nerves passes with them through the cavity of the cranium and that of the spine. It is very distinct in these cavities, because it is not surrounded there with cellular texture, but only with the arachnoides, which may be raised with great ease; instead of using the different preparations that Reil mentions for the purpose of separating the coat of the nerves from the cellular texture, it is infinitely more convenient to examine this membrane upon the last nerves of the spine, which are, as we have seen, remarkably long.

_Action of certain substances upon the nervous coat; its resistance, &c._

Without the osseous cavities, the nervous coat embedded in the cellular substance, adheres to it strongly, but appears evidently to be of the same nature as in the interior. We are ignorant what its nature is, whether it is the same as that of the pia mater, of the medulla, of the tuber annulare and its dependancies. It appears to have great affinity with the cellular texture. It is transparent and consequently free from the colour of the nerves; hence why, when they have been deprived, by alkalies, of their pulp, they lose a great part of their whiteness.

The coat of the nerves is one of the parts of the animal economy which are hardened with the greatest ease, especially at the instant the nerves are immersed in an acid slightly concentrated, particularly the nitric and sulphuric. I have not observed in any other texture this phenomenon in so remarkable a manner; the nerve is suddenly diminished in size and twisted in different directions; now we shall see that the medullary substance is in no way concerned in this phenomenon. The action of boiling water produces an analogous effect; by it the nerve is wrinkled, contracted and hardened; then, after the ebullition has continued for a certain time, it gradually becomes soft, and its whitish colour is changed to a sort of yellowish tint, very different from that of boiled tendon or aponeurosis. The action of the acids continued for some time, produces an effect analogous to that of ebullition. To the sudden hardening like horn which the nerve undergoes, soon succeeds a softness so great that at the end of a short time it is easily moved under the finger, and afterwards becomes partly dissolved.

The alkalies do not produce the horny hardening in the nervous coat any more than in any other texture of the living economy; neither do they dissolve it. Hence, undoubtedly, why Reil, having macerated for some time a portion of nerve in soap-boilers lie, was able to separate accurately the nervous coat from its medullary substance.

The action of water upon the nervous coat produces a phenomenon that is exhibited by few others of the animal textures. Far from softening it immediately and then reducing it to pulp, it seems in the beginning to increase its consistence. A nerve soaked in water becomes there harder and more resisting, and this state, at the ordinary temperature of cellars, continues for a month and a half, and even two months. It is only at the end of this time and frequently longer, that the texture of the nervous coat is gradually softened, and broken, and finally ends by being diffused like other macerated textures. I have not repeated this experiment in a very warm temperature, which has always succeeded in that of winter and spring.

The coat of the nervous filaments has a very great resistance, because it is, in proportion to the medullary substance that it contains, infinitely thicker than the membranous canal of the spinal marrow. It is thus that the proportion between the thickness of the vascular parietes and the fluids they contain, is much less in the great trunks than in the small branches; the fluid considerably exceeds the solid in the first, there is at least an equality in the second. Thus a very small nerve would support a much greater weight than the spinal marrow. I believe that among the textures which are arranged in filaments or in elongated tubes, this and the arterial, next to the fibrous, afford the greatest resistance; they surpass the venous, the muscular, the serous, &c.

_Medullary substance; its origin._

This substance occupies the interior of the nervous canal, in the same way as the substance of the spinal marrow fills the canal formed by the pia mater. This medullary substance is whitish, as that of the brain and spinal marrow; it gives the nerve its colour. It is in much greater proportion in the optic nerve than in any other; it is found exclusively in that part of it posterior to the junction of the two, as well as in the olfactory. It is so abundant in the auditory that it seems to form a great part of it. In general, I think at the origin in the osseous cavities, it predominates over the nervous coat, but in the course of the nerve, the nervous coat is the greatest. Hence the greater degree of resistance of the nerves in the second, compared to what they have in the first.

This substance appears to be continuous with the medulla of the brain, the tuber annulare and its dependancies, and the spinal marrow. I think, no one can deny this continuity with the origin of the optic and olfactory nerves, in which more of this medullary substance is found than in the other nerves. In the auditory, also, it is very apparent; in the spinal marrow, by scraping this white substance from the internal surface of the pia mater, so as to leave the nerves adhering to this membrane, we see evidently at the place where these nerves go off, that there is an elongation penetrating their nervous coat.

_Comparison between the medullary substance of the brain and the nerves._

What is the nature of the medullary substance of the nerves? I have endeavoured to institute a comparison between it and the cerebral substance; there is considerable analogy under some points of view and some difference under others.

1st. Submitted to drying in the open air, in small slices to prevent putrefaction, the white substance of the brain becomes yellow, and acquires considerable consistence. The nerve dried becomes yellow also, hardens and contracts. These changes are undoubtedly owing in part to its coat. The proof of this is that if we dry the covering that the pia mater furnishes to the spinal marrow, a covering that has great analogy to the nervous coat, the new qualities it acquires are very analogous to those of the dried nerves. But this does not prevent the medullary substance of the nerve from contributing also to the yellow colour by the evaporation of its watery part. I will make, in regard to this, a remark that I think interesting; it is this, that water has an influence upon the whiteness of a number of textures which become yellow or greyish by its subtraction, and are whitened again by its addition. Thus we have the power of making yellow, by drying, all the fibrous organs, the skin, &c. and of restoring them afterwards to their primitive colour. Thus also the serous surfaces, the cellular texture, &c. that have become greyish from drying, regain their whiteness when immersed in water, if they have not been long dried. The epidermis of the sole of the foot and the palm of the hand turns from grey to white, when it has been immersed for some time in water.

2d. The cerebral substance and that of the spinal medulla easily become putrid when submitted to the combined action of water and air; they become of a greenish colour and have acid sufficient to redden blue paper. Of all animal substances, I think they exhibit this phenomenon the soonest. The nervous medullary substance, on the contrary, resists putrefaction much longer. The nerves are among the slowest of all the parts of the animal economy to become putrid. During life they are often found untouched in a gangrenous limb, in the middle of an abscess, &c. In a dead body which is putrid, they preserve their whiteness and consistence, while the other parts are black and soft. I have observed that the water in which the nervous system has been macerated has but little odour, but that that in which the brain has been macerated is fœtid. These phenomena clearly would not take place if the medullary substance of the nerve became as easily putrid as that of the brain. It is manifest, however, that it is especially to the nervous coat, that the nerves owe this sort of incorruptibility; for I have observed, that the optic nerve, in which the medullary substance predominates, and the olfactory and auditory, which are abundantly furnished with it, become putrid sooner than the others. I have remarked also uniformly, that whilst the white substance of the spinal marrow becomes putrid, its covering remains untouched.

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.

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.