Page images
PDF
EPUB

guides. By thus extending his observations to both these organs, he gained the important advantage of comparing with each other various nerves which differed essentially in the number and structure of their roots, and of elucidating the functions of the one kind by contrast with the others.

If a Spinal nerve be taken anywhere in its course through the body, and traced backwards to its source in the Spinal cord, it will be found that, when it gets near the organ, it splits into two sets of fibrils, of equal size, called its "roots." On further examination, each root will be seen to dip into a division of the cord, circumscribed by distinct boundaries, termed a "column.” One of these columns being situated at the back of the organ, the root connected with it obtains the name "posterior;" and a rounded body, of a reddish hue, called a "ganglion," is formed upon this posterior root. The fibrils of the other root are directed forwards and lost in the substance of a column situated in front, whence it is called "anterior;" and is distinguished from its fellow in not having a ganglion.

Encouraged by the observation of these marked differences in the roots to suppose that his theory was well founded, and that the one would be found to have a distinct function from the other, he felt justified in putting his views to the test of experiment, by exposing the spinal cord with its roots in a living animal, and dividing, or irritating them, in succession. Accordingly he proceeded to perform that operation; and the results realised his anticipations. They proved decidedly that the "anterior" root was distinct in its functions from the "posterior."

But here it must be stated that the author was not satisfied to rest his conclusions upon the particular functions of these roots on experiments confined to them alone. His final views were formed by associating the results obtained through them with others derived from experiments, presently to be described, on the nerves of the Brain.

The principal fact in regard to the spinal nerves, which he was sure of having fully established, was that the power of giving motion is exclusively the property of the "anterior" roots. Each time these were pinched, a convulsive action of those muscles which correspond to the distribution of the nerves experimented upon took place, leaving no doubt of the cause of

the movement: but a similar effect could not be produced by any amount of irritation applied to the "posterior" roots.

It might have been thought that when the author had thus decidedly shown that the "anterior" roots bestowed motor power, he would have concluded at once that the remaining function of the nerves, sensation, belonged to the "posterior" roots. But such was not his course of proceeding. He was convinced, that for determining the seat of a power of the nature of sensation, direct experiments on the roots themselves could not alone be depended upon. To comprehend the grounds of his want of confidence, we have merely to reflect on the peculiar character of the experiments. Necessarily, they are of a violent and painful description. They involve, first, the making of long and deep incisions through the skin and muscles of the back; the next proceeding is that of forcibly breaking into the narrow bony canal situated in the centre of the vertebral column; lastly, to expose the spinal cord and roots, the membranous sheath which immediately invests them has to be extensively opened. Now, the unavoidable consequence of all that severe operation is, that the animal, the subject of it, is stunned, stupefied, and terrified. It is, therefore, in a condition altogether unfit for drawing distinctions as to its capacity of feeling, or not feeling, in connexion with manipulations made by the experimenter on the particular roots at the depth of the wound. Accordingly, before the author drew his final conclusion concerning the function of the "posterior" root-which was decidedly that it conferred sensation-he had sought for and obtained evidence of a perfectly reliable kind to confirm its truth; his judgment was principally based on corroborative proofs derived from experiments on nerves of the Brain, next to be described.

When he proceeded to investigate the nerves of the Brain, his attention was directed chiefly to the two nerves which have the most extensive distribution of any in the head, viz., the "Portiodura" and the "Fifth."

Before describing the experiments made on these two nerves, a brief account of the structure of the roots of each must be given; when it will be perceived that it would not have been possible to have selected from the whole body any other nerves better calculated to prove the soundness of his theory, and to

demonstrate the distinction between the nerves of motion and sensation.

The Portio-dura is distinguished for its arising from the brain, in remarkable contrast with the spinal nerves, by a single root alone. In common with a series of other nerves, (to which I shall presently refer,) it comes off from a circumscribed portion of the base of the brain, by a root upon which there is no ganglion. Having pierced the skull, it emerges on the face, in front of the external ear, and lies there almost directly under the skin. During its course it forms no important connexion with any other nerve, so that it is as simple in its anatomical structure at that part as when it had just arisen from the brain. The nerve now subdivides into numerous branches: these take a leap, as it were, across a particular class of muscles, those of the jaws, to avoid them; and they are eventually distributed, in the fore-part of the face, to the muscles which move the features.

The Fifth is characterised by being the only nerve among those of the brain which arises, like the spinal nerves, by two distinct roots, each from an appropriate part of the organ; and not only are its roots double, but upon one of them is formed a "ganglion" exactly like the body of the same name on the posterior roots of the nerves of the spine. Yet, although the analogy here indicated cannot be doubted, a difference is to be observed in the roots of the Fifth, which adds much to the interest of examining its functions. In the spinal nerves, without exception, the two roots are of equal dimensions; consequently, all the branches consist of the same number of fibrils of each, and no distinction can be drawn between them. But in the Fifth, the root upon which the ganglion is formed is fully five times greater in size than the one which has no ganglion. Hence it follows that, in the distribution of the branches of the whole nerve, a large proportion belong exclusively to the "ganglionic" root, and a few only are composed of the two roots joined together. It is also observed that the branches prolonged directly and simply from the large root, course to all the surfaces of the head, the skin and sensitive membranes, where no muscles exist,-while those which contain the ils the lesser root can be traced to the group of muscles which the Portio-dura had passed by, viz., the muscles of the jaws.

1

The experiments performed by the author on the Portiodura, which goes to the features, were in their results most decisive. To expose the nerve, he had to make only a small incision, scarcely larger than that for venesection; and when he cut it across, the effect was instantly visible. All the muscles corresponding to the distribution of the nerve were at once arrested in their motion-paralysed; but the sensibility of the skin was not in the smallest degree impaired. Among other animals on which he performed the experiment was the monkey; selected on account of the mobility and activity of his features. Before the operation, the creature was, of course, full of grinnings and grimaces at the liberties taken with him: the moment that the Portio-dura was severed, although his anger and jabberings did not cease, his face became passive and expressionless, like a mask. It was thus incontestibly proved that the only function with which this nerve is endowed, is the power of giving motion to the muscles.

The experiments which he next undertook were upon the Fifth. And he chose, first, those branches that emerge upon the face, at three distinct points, to supply the same parts to which the Portio-dura is sent. Although he doubtlessly perceived that, as motor power simply was conferred by that nerve, the remaining function, sensation, would, almost certainly, be bestowed by the only other distributed to the part, the Fifth, yet he subjected the branches to the necessary experiments.

These branches have a particular interest attached to them, from the mode of their origin. Each comes off directly from the large, ganglionic root; and each pursues its course to its appropriate part of the face, without forming a connexion with any other nerve. The individual branches are, therefore, true representatives of the root from which they arise. Again, when they arrive at their destinations, they are situated quite superficially, being covered alone by skin; and they can be exposed for experiment with the utmost facility. Accordingly, experiments performed on these branches are essentially the same in value for determining the function of the ganglionic root, as if they had been made directly upon the root itself within the cranium.

When these branches were cut across in a living animal, the result expected was obtained. It was shown that their property

b

was to bestow sensation; and that, in correspondence with their arising simply from the single root, the "ganglionic," they had no other function, and could not give motor power. When divided, the skin of the animal could be freely pinched and pricked without drawing forth any signs of feeling or pain. Yet, although the part appeared thus dead, the movements were not in the smallest degree directly affected: they were preserved through the influence of the Portio-dura, which was entire.

The experiments now remaining to be performed were upon those branches of the Fifth which are composed of the lesser, "non-ganglionic" root, combined with some fibrils of the larger root, and which are distributed wholly to the muscles of the jaws. When these compound branches were cut across in a living animal, two effects were instantly produced: first, sensation was destroyed in the surfaces to which the fibrils of the larger root were distributed; secondly, all power of motion was immediately lost in the muscles of mastication. The jaw-bone dropped, and could not be raised to bring the teeth together. When the cut ends of the nerve were pinched by forceps, the paralysed muscles acted with spasmodic suddenness; the jaw closed with a snap; and the fingers of the incautious assistant being between the teeth, received a sharp bite.

The name which the author chose for expressing the double nature of the functions of the Fifth, and the peculiar appropriation of its motor root, was the "Nerve of Sensation and Mastication."

By these joint experiments on the two nerves of the brain, the Portio-dura and Fifth, every proof that could be required was furnished to demonstrate, in the most satisfactory manner, that the nerves of motion are distinct from those of sensation; that the distinction bears direct relation to the roots by which they arise respectively from the brain; and that when a nerve is found to possess both functions conjointly, it is a sign that it is really double in structure-composed of fibrils, one set of which come from an appropriate part of the brain that bestows motion, and the other from a different part that confers sensation.

It now rests with me only to point the reader's attention to the value of the observations on the two nerves of the brain just described, in elucidating what was obscure regarding the func

« PreviousContinue »