Part 14

All anatomists have heretofore considered the nervous system in an uniform manner; but if we reflect a little upon the forms, the distribution, the texture, the properties and the uses of the different branches that compose it, it is easy to see that they should be referred to two general systems, essentially distinct from each other, the one having the brain and its dependancies for its principal centre, and the other, having the ganglions. The first belongs especially to animal life; it is on the one hand the agent, that transmits to the brain the external impressions that are to produce sensations; and on the other it serves as a conductor of the volitions of this organ, which are executed by the voluntary muscles to which it goes. The second, almost every where distributed to the organs of digestion, of circulation, of respiration, of the secretions, belongs more particularly to organic life, in which it performs a part much more obscure than that of the preceding one. Neither is strictly confined to the organs of either life. Thus the cerebral nerves send some branches to the glands, to the involuntary muscles, &c.; and the nervous system of the ganglions have ramifications in the voluntary muscles. It is from the general arrangement, without regard to particular exceptions, that the division of the two nervous systems is founded, between which I shall not draw a parallel here to show their difference, because the description of each will be sufficient to do this.

The nervous system of animal life is exactly symmetrical, like all the organs of that life. The brain and spinal marrow, which are the double origin of this system, have this character in a remarkable degree. Nerves precisely similar go from them; hence the name of pair, by which is designated the double, corresponding trunk, a name, that we should not be able to employ commonly in the system of ganglions. There are then really two nervous systems of animal life, the one right, the other left; the median line separates them. Their distinction is apparent not only from dissection, but from their diseases. At one time exactly one half of the body is deprived of motion, and the whole nervous system of that side remains passive, the other retaining its ordinary activity; at another, one side only has an unnatural energy and becomes the seat of convulsions, while the other remains calm. In both cases, sometimes the phenomenon is general; often it is limited to a greater or less number of lateral organs; but always there is an evident separation between the two nervous systems, the right and left. The kind of partial paralysis, of which I just spoke, and the principal character of which arises from the symmetry of the nervous system of animal life, is wholly different, as it regards this character, from that in which the lower parts of the body are deprived of motion in consequence of a fall upon the sacrum, or any other analogous cause.

The relations of size of the nervous system with the brain are in man and most quadrupeds, in an inverse proportion, as has been observed by Soemmering. In man the brain is much more voluminous than in the others, who have nerves larger than his. It is easy to prove this assertion, in all the animals that we commonly employ for our experiments; in fact, small dogs are used, on account of the size of their nerves in very delicate experiments upon sensibility. This difference is a striking proof of the superiority of man, as it respects the intellectual phenomena, which are all referable to the encephalic mass. On the other hand, many animals are superior to him as it respects motions and the four senses of taste, of smell, of hearing and seeing. Observe however that he surpasses them all in the perfection of the fifth sense, viz. that of touch. Why? Because this sense is entirely different from the others, is consequent to them, and corrects their errors. We touch, because we have seen, heard, tasted and smelt. This sense is voluntary; it supposes reflection in the animal that exercises it, the others do not. Light, sounds, &c. strike the respective organs without any effort of the animal; but he touches nothing without a preliminary act of the intellectual functions. It is not then astonishing, that the perfection of the organs of touch and the great development of the brain, should be in man, in the same proportion, and that in those animals, in whom the brain is more contracted, the touch should be more obtuse and the organs less perfect.

ARTICLE FIRST.

EXTERNAL FORMS OF THE NERVOUS SYSTEM OF ANIMAL LIFE.

I shall consider these forms, 1st. in the origin; 2d. in the course; 3d. in the termination of the cerebral nerves.

I. _Origin of the cerebral nerves._

The word origin should only be understood in relation to anatomical arrangement. In fact, the nerves are formed at the same time as the brain; they are rather organs of communication with this viscus, than elongations of it. If we take a view of the functions of one part of the nervous system, we shall see that the termination is at the brain, and the origin is upon the surface. Is it not said that the nerves go towards this or that part, that the arteries take this course, wind about, &c.? These are only metaphorical expressions, the least reflection will determine their meaning.

The nerves of animal life derive their origin from three principal portions of the encephalic mass; 1st. from the cerebrum; 2d. from the Tuber Annulare or Pons Varolii and its elongations; 3d. from the spinal marrow; the cerebellum gives origin to none. This circumstance, which, we ought not to lose sight of in the examination of the functions of each part of the brain, and which will perhaps hereafter elucidate these functions, is undoubtedly sufficient to make us appreciate the opinion of many physicians of the last age, which placed in the cerebellum the source of involuntary motions, and attributed the voluntary to the cerebrum.

The cerebrum furnishes but two nerves, the olfactory and optic; these are remarkable, 1st. in this, that their adhesion is very strong at their origin with the brain, and that by raising the pia mater, we cannot remove them; 2d. in this, that their softness is greater than that of most other nerves.

The tuber annulare and its elongations, as well those that go to the cerebrum and the cerebellum, as that which begins the spinal marrow, furnish the motores communes of the muscles of the eye, the pathetic, the origin of which, though posterior, is evidently derived from the tuber annulare, the trigemini, the motores externi of the eye, the facial, the auditory, the par vagum, the glosso-pharyngeal and the great hypo-glossal. All these nerves are distinguished by different characteristics. 1st. As the medullary substance is every where exterior to the eminences from which they arise, all appear manifestly to be continuous with this substance. 2d. Almost all begin by many filaments separated from each other; sometimes, as in the trigemini and par vagum, these are very numerous. The others arise, some by one filament and others by two. 3d. Except the auditory nerve, all have a greater consistence at their origin than these last. 4th. They adhere but little to the corresponding cerebral portion, so that they are almost always raised in detaching the pia mater; thus it requires great precaution, to prevent breaking the nerves from the brain when it is taken out of the skull. The adhesion of the pathetic, the motores communes and the facial, is particularly weak. We should almost say, from a slight examination, that there was only contiguity.

The spinal marrow gives origin to thirty or thirty-one pair of nerves, viz. eight cervical, twelve dorsal, five lumbar, five or six sacral, and to a nerve that enters the cranium and goes out of it under the name of spinal. The following are the characters of these nerves at their origin. 1st. They are continuous, like the preceding, with the medullary substance. 2d. They all arise by two cords, one anterior, the other posterior. These cords derive their origin from many filaments, placed above each other, most usually separate and always distinct. 3d. The adhesion is much stronger at the origin of these nerves than at that of the preceding, a circumstance that depends upon a cause that will be hereafter pointed out. 4th. The consistence of the spinal nerves is also very evident in their canals.

From what has been said, it is clear, that the nerves do not arise deep in the cerebral substance, at least in an apparent manner, but take their origin from its external surface. Many physiologists however, have admitted an origin more remote than can be proved by examination. They have believed that the nerves of one side arise from the opposite, and that each pair cross each other not only in the brain, but also in the spinal marrow. This opinion is founded upon a singular phenomenon, viz. this, that paralysis almost always takes place on that side which is opposite to the affected side of the brain, a phenomenon that is frequently noticed in diseases and proved also by experiments, as has been shown by Lorry. On the other hand, it is said that convulsions are seated in the side corresponding with the injured side of the brain; but this fact is more uncertain than that of paralysis, which is incontestable. I do not believe that with our present knowledge we can explain this last, and the anatomical opinion pointed out above, is contradicted at the first sight.

I will make but one observation upon this singular phenomenon, and that is, that it particularly concerns the nerves of motion, and hardly ever affects the nerves of sensation. In fact we know, that in wounds of the head, in consequence of apoplexy, &c. one eye, one ear, one side of the tongue, one nostril, do not become insensible, as the muscles of one side cease to move. We do not suddenly become paralytic on one side as it regards sensation, as we do as it respects motion in hemiplegia. Experiments cannot elucidate this, for it is impossible to perceive the alterations of sensibility as we do those of mobility. However, by compressing the brain of a dog, and thus rendering him paralytic on one side, and then shutting each eye separately and alternately, to see if he distinguished objects, and afterwards by presenting in turn to each nostril volatile ammonia, or other pungent substances, I have not seen, as it regarded the sensibility, an alteration corresponding with that which the mobility experienced. We often observe in man a discordance in the organs of sense. One ear hears better than the other, one eye sees further, &c.; hence false hearing, hence a species of strabismus, &c.; but the cause of these discordances appears to reside in the organ itself, and not to be connected with the brain.

Moreover, it does not appear that each hemisphere always corresponds necessarily with the nerves of motion of the opposite side. In fact, we often see on the right side effusions or injuries of the cerebral substance, without any alterations of motion on the left, and vice versa.

The following are the cerebral membranes that are found at the origin of the nerves; 1st. the dura mater forms for them a kind of canal in the fissure through which they go out, then it quits them entirely, and is partly lost in the cellular texture, and the remainder is reflected upon the edges of the opening and continued with the periosteum. The optic nerve is the only exception to this; it is accompanied in its whole course by a fibrous canal, which goes even to the sclerotic coat, which in this way, communicates with the dura mater. 2d. The tunica arachnoides surrounds every nerve at its origin with a fold formed oftentimes in the shape of a tunnel, the broadest part of which is at the origin. By carefully raising the brain, or by opening the dura mater of the spinal canal, we very easily discover this fold, which is continued to the osseous opening, through which the dura mater enters, it is then reflected upon the surface of this membrane corresponding with the brain, forming a sac between it and the nerve. Sometimes, as in the optic nerve and the motor externus, it penetrates the fibrous canal of the dura mater, and accompanies the nerve to the middle of the canal, which, consequently, is lined in part by the tunica arachnoides, and partly by the cellular texture. 3d. The pia mater is used in a manner that is difficult to understand, and which has not as yet been well explained. I shall speak of its continuity upon the nerves, in treating of their peculiar membrane.

The nerves go over a more or less considerable extent before going out of the cranium or the spinal canal. 1st. The two that arise from the cerebrum are much longer within than without. 2d. Among those of the tuber annulare and its dependancies, the pathetic nerves are the only ones that remain any length of time in the cranium before they go out of it, and that have a greater extent there than externally; all the others go out almost immediately. 3d. The nerves of the spine have a much greater extent when examined lower down. Above, they become directly external; below, they are six inches long in the canal, and consequently pass many foramina before they arrive at their own; hence it happens, as has been observed by Jadelot, that if we avail ourselves of the spinous processes, on account of their prominence, to judge of the origin of the nerves in the application of the moxa, it is necessary, in the neck, in order to act upon a point corresponding with the origin of any nerve in particular, to take nearly the spinous process of the vertebra that corresponds numerically with the pair that we have in view, whilst in the loins it is much above this vertebra that the application should be made.

The direction of nerves at their origin is also very variable. At the cerebrum and the tuber annulare, there is no general arrangement. But in the spinal nerves, this direction, almost perpendicular to the marrow above the cervical region, always becomes more and more oblique down to the end of the lumbar region. These three things, viz. the length in the canal, the size and oblique direction of the spinal nerves, successively increase from above downwards in a gradual manner, with some exceptions as to size.

Each pair of nerves, in going from the cerebrum, the tuber annulare or its dependancies, and the spinal marrow, diverges in the two trunks which form the pair. The olfactories alone converge, and the spinal run nearly parallel.

II. _Course of the cerebral nerves._

The nerves exhibit different arrangements at their exit from the osseous cavities that contain their origin.

_Communication of the cerebral nerves at their exit from their osseous cavities._

1st. The two nerves of the cerebrum, go without communicating with any other, to their respective destination. 2d. Those of the tuber annulare and its dependancies begin to have communications, which are much more evident when examined inferiorly. Thus the par vagum and the great hypo-glossal nerves, send in going from their respective foramina, numerous filaments to the neighbouring organs, whilst above, the motores communes, the pathetici and even the trigemini shew this arrangement less evidently; the auditory nerve does not communicate with any other. 3d. The communications of the nerves of the spine are more evident at their exit, especially in their anterior portion. The deep cervical plexus, the brachial, lumbar, and sciatic, arise from these communications, which are not so visible in the intercostal nerves.

These kinds of plexuses have a particular arrangement. They are formed in the following manner; each nerve, at its exit from the foramen, sends a branch above and below, and also receives one; so that the cords that succeed those that go from the foramina, arise from two or three of these. These second cords, in dividing, send branches above and below, receive them and form third cords; so that in the brachial plexus, for example, when the nerves cease to communicate thus, and are divided into separate trunks, that each may go to its destination, it would be impossible to say correctly from which pairs they arise. It would require a very tedious dissection to ascertain precisely from what pairs come the median, the cubital, &c.

It is this consideration that has induced me not to describe the nerves of the spine as it is usually done, that is, as going from such or such pairs. I describe at first in each region the plexus that the nerves form there in going out of the spine; thus, I expose before the cervical nerves, the deep cervical plexus, before the brachial the brachial plexus, and before the lumbar and sacral the plexuses of the same name. The general arrangement, the form, the relations of these plexuses being known, I pass to the description of the nerves that go from them before, behind, without, within, &c. without regard to the pairs of nerves that come through the foramina. This method has appeared to me, moreover, to be extremely convenient for students. Nothing, for example, is more complicated than the description of the cervical nerves, by referring them to the pairs that first furnished them. But understand well at first the deep plexus, arising from the anastomoses of these pairs at their exit; afterwards class the nerves, 1st. into internal, which go to the great sympathetic; 2d. into external, which are distributed upon the acromion and the triangular space, bounded in front by the sterno-mastoideus and behind by the trapezius; 3d. into anterior, which, winding upon the sterno-mastoideus, form there with the branches of the facial a kind of superficial plexus; 4th. into posterior, which go either to the occiput, or to the posterior muscles of the neck; 5th. into those that go inferiorly, as the diaphragmatic, as those that communicate with the nervous branches of the hypo-glossal, &c. &c. In this way, you will easily retain all the nervous distributions, because you will have one point to which your memory will refer them all, and not to as many centres as there are pairs.

_Internal communications of the nervous cords._

It is not only at their exit that the spinal nerves thus communicate. The different cords of which each nerve is formed have precisely the same arrangement, as may be easily seen in the great trunks, as in the median, the cubital, the radial and especially the sciatic. By separating the different cords of these nerves, we see that they are not only in apposition longitudinally, but that they send numerous filaments to each other. These communications do not resemble those of the arteries, in which there is always continuity between the communicating branches. Here there is only contiguity, and each of the cords forming the nervous trunk is, as we shall see, composed of filaments; now it is these filaments that frequently go from the cord to which they belong to a neighbouring one; so that after a short distance, the cords that began the nerve are not composed of the same filaments as those that finish it; the whole becomes mingled together in the course of the nerve. Thus the cords of the branches of the brachial plexus, at its origin, are not arranged like those of the branches that terminate it. For there is this difference between the very evident plexuses formed by the nerves themselves and those that are less evident formed during their course in their interior even, viz. that in the first, it is the cords that go off and form the interlacing, and in the second it is the filaments. I once amused myself in tracing the filaments of the sciatic for a short distance; now those which composed above the external cords, were found for the most part below in the cords of the centre.

This remark proves that there are not nervous cords destined to sensation and others to motion, and that if the same nerves do not serve the double use, the difference is in the filaments, and not in the cords.

In the interior of the vertebral canal, in which the nervous cords are much insulated, for the want of cellular texture, the filaments that compose them do not thus communicate with each other; there is not, as without, a plexus in the interior of the nerve. This is remarked particularly at the extremity of the canal, where the nerves run a long course, as I have before said.

The communication of the nerves at their exit from their osseous cavities is so general, that under this point of view it may be said that they form on every side a kind of organ every where continuous, an organ to which the optic, the olfactory, and the auditory nerves only are strangers.

Besides, these kinds of communications, which are all made by juxta-position, do not appear to have much influence upon the functions of the nerves. Each of their cords, though belonging in its course to many different trunks, can perform its functions in an insulated manner; so can each filament, though concurring in its course to form many cords of the same nerve.

I would observe with regard to this, that it is necessary to distinguish accurately these communications from anastomoses, in which two nervous filaments coming in an opposite direction, are confounded and identified with each other, which is seen between those of the facial, the sub-orbitary, the mental, &c.

_Nervous trunks._

After having thus communicated at their exit, the nerves separate from each other and go towards the different organs. They form at first considerable trunks, which pass through the great cellular interstices and go over a greater or less extent. The form of these trunks is sometimes flattened as in the sciatic; but it is most commonly rounded; but the form does not affect nervous action, for the nerves that are naturally round, when flattened by a tumour, perform their functions as usual. In general, whenever it does not interfere with her design, nature chooses the round form for the organs of animals. I would observe also, that this form requires a system generally diffused, and destined to fill up the spaces that necessarily exist between round organs; this system is the cellular. It would be infinitely less necessary, if the form of our organs was square, because there would be less space between them.

The nervous trunks are of different length. Those of the extremities hold the first rank in this respect, because the extremities being very distant from the origin of the nerves, these trunks must of course go over a certain extent before distributing their filaments. In the trunk and the head, on the other hand, as the organs are presented immediately to the nerves that enter them, the division into branches is immediate, and the trunks are very short.

The nervous trunks are sometimes accompanied by a corresponding arterial and venous trunk, as the brachial, crural trunks, &c.; at other times, as the sciatics, and those of the par vagum, they go separate.

_Of the nervous branches, &c._

As the trunks advance, they furnish here and there different branches; these give out smaller ones, which send off those that are still smaller, from which arise the last divisions. All these different divisions take place at very different angles. The acute angle is the most common. It is not a real origin, but merely a separation of many cords united, that forms the branches, of one or two of these for the smaller ones, of one cord only for the still smaller, and of separate filaments for the last divisions. Thus this separation is made more or less high, in different subjects. The place where it happens is never exactly determined.