General Anatomy, Applied to Physiology and Medicine, Vol. 1 (of 3)

by Xavier Bichat

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.