Different nerves for different offices.

posterior root is a nerve of sensation, and the anterior a nerve of motion. It is a mere matter of convenience that they unite, and are mingled together in the same sheath, for they are to be distributed in the same parts. In and about the face the nerves of motion and sensation are kept for the most part separate, as before stated, merely because it would be no convenience in any case to put them together in one sheath.

241. But not only are there different nerves for sensation and for motion, but there are also different nerves for different kinds of sensation. Thus, in the eye, the optic nerve which transmits the impressions from the images formed on the retina, as will be shown in the Chapter on the Eye, is wholly separate from the nerve by which any pain or irritation is felt in this organ. The latter is called a nerve of common sensation the former a nerve of special sensation. So in the nose, the nerve that takes cognizance of odors is a different one from that by which irritation on the same membrane is felt. The snuff-taker smells the snuff with one nerve, and feels its tingling with another.

242. The nerves devoted to one kind of sensation can not in any case perform the function of those of any other kind. Each nerve is fitted for its own peculiar office, and has for this its own peculiar susceptibility. Thus, the nerve of touch is insensible to light, and, on the other hand, the nerve of vision is insensible to touch. If, therefore, the nerve of vision be paralyzed, but the nerve of common sensation in the eye be unimpaired, although there is no seeing, the eye is as sensible to irritation as ever. On the other hand, if the nerve of vision be unimpaired, and the nerve of common sensation be paralyzed, as sometimes happens, the individual can see, but he has lost the sentinel that stands guard over the eye, and by its warning of pain keeps it from injury. What, therefore, is flying in the atmosphere may lodge in the eye, and though it produce no pain, it will excite inflammation by irritating the capillaries. The eye in some of these cases is destroyed, by the inflammation which thus arises from the loss of sensibility to the touch. When the nerve of common sensation is in a healthy state, the moment any thing gets into the eye great pain is produced, and the tears flow and the eyelids are in constant motion; and if by these instinctive means, as we may term them, the irritating substance is not removed, other means are at once resorted to. But when this nerve is paralyzed, although the irritating substance produces no pain, it

Different degrees of sensibility in the various parts of the body.

gradually causes inflammation in the delicate vascular texture of the eye. Pain, then, in this case, as well as in every other, is a safeguard against danger. The part is endowed with an acute sensibility to touch, because it is needed as a sentinel in a part so delicate and yet so exposed.

243. This leads me to remark, that the different parts of the body are endowed with different degrees of sensibility, according to their necessities, in relation to the warning of danger. Accordingly, the skin is the most sensitive part or organ of the body, that it may warn at once of the approach of danger; while the internal parts have much less sensibility, some of them none. In the performance of operations, therefore, the great suffering is in the cutting of the skin. There is very little sensibility in the muscles, and there is none in the bones. The following fact illustrates the use of the sensibility of the skin in the prevention of injury. A man who had lost all sensibility in his right hand, but retained the power of motion, lifted the cover of a pan when it was burning hot. Although he was not aware of any effect at the moment, the consequence was the loss of the skin of the fingers and of the palm of the hand, laying bare the muscles and tendons. If the sensibility had not been lost, that is, if the nervous tubuli which transmit sensation had not been paralyzed, the warning of pain would have been instantly given to the brain, and orders would have been sent to the muscles to relax their grasp of the cover; and so rapid are these transmissions, that the cover would have been dropped soon enough to prevent any great amount of injury from being done.

244. Although there is so little sensibility in the internal parts in their healthy condition, yet when they become inflamed. they become sensible of pain, sometimes acutely so. Thus, an inflamed bone is the seat of severe pain; and the tendons, although nearly insensible ordinarily, become very painful when inflamed, as any one that has a deep-seated felon can testify. The question as to the cause of this change of sensibility I will not stop to discuss, but that there is a benevolent object in it is very manifest. If inflammation caused no pain in such parts, it might go on to a destructive extent without the person's being aware of the danger, and therefore without his applying for medical means.

245. It was formerly supposed that a nerve must of course have an exquisite sensibility. But there is no sensibility in nerves devoted to motion. Neither is there any in the brain

Insensibility of the heart to the touch. Respiratory nerve of the face.

itself. Portions of it can be cut off without producing any pain. The heart, too, is insensible to the touch. A case proving this fell under the observation of Harvey, the discoverer of the circulation of the blood. A young nobleman, from an injury received in a fall, had a large abscess on the chest, which occasioned such a destruction of the parts, as to leave the lungs and heart exposed. Charles I., on hearing of the case, desired to have Harvey see it. When," says Harvey, "I had paid my respects to this young nobleman, and conveyed to him the king's request, he made no concealment, but exposed the left side of his breast, when I saw a cavity, into which I could introduce my fingers and thumb; astonished with the novelty, again and again I explored the wound, and first marveling at the extraordinary nature of the case, I set about the examination of the heart. Taking it in one hand, and placing the finger on the wrist, I satisfied myself that it was indeed the heart which I grasped. I then brought him to the king, that he might behold and touch so extraordinary a thing, and that he might perceive, as I did, that unless when we touched the outer skin, or when he saw our fingers in the cavity, this young nobleman knew not that we touched his heart!" This absence of sensibility in the heart is not because it is not well endowed with nerves. It is well endowed, but it is with nerves which are devoted to another purpose. They are nerves of sympathy, which establish a connection with every part of the body, making this organ to be so easily affected by motion, by disease, and by every passing emotion in the mind.

246. In the face we have an example of different sets of nerves for different classes of motions. All those motions that are used in the expression of the countenance are associated together by a certain nerve. This nerve has nothing to do with other motions, as mastication. Other nerves are provided for them. Sometimes this nerve of expression is paralyzed on one side. The result is, that while the individual can masticate equally well on both sides, he can laugh, and cry, and frown, only on one side, and he can not close the eye on the side affected. In Fig. 82 is a representation of this condition of things. The left eye can not be closed by any effort, and the left side of the face is wholly devoid of expression. This nerve of expression is often paralyzed by itself, the other nerves in the neighborhood, both nerves of sensation and of motion, being entirely unaffected. This nerve has been called the respiratory

nerve of the face, because it controls motions which are connected with the movements of respiration. If you observe how the various passions and emotions are expressed, you will see that there is a natural association between the muscles of the face and those of the chest in this expression. This is very obvious in laughing and in weeping. But this association can be effected only through nervous connections. And these connections in this case are very extensive and intimate. When the nerve of expression, or facial respiratory nerve, is paralyzed, all the motions of the face connected with the respiration are absent. Though the individual may sob in weeping, or send forth the rapid and successive expirations of laughter, yet the face on the side where the nerve is paralyzed will be perfectly quiescent. So, too, those movements of the nostrils which are sometimes used in expression, can not be performed. Sneezing and sniffing up can not be dono

Nerves of the eye. Paralysis affecting different nerves.

on the affected side. Neither can the individual whistle, because a branch of this nerve goes to the muscles at the corner of the mouth, which are therefore disabled, Sir Charles Bell, in cutting a tumor from before the ear of a "coachman, divided this branch of the nerve. Shortly after, the man thanked him for curing him of a formidable disease, but complained that he could no longer whistle to his horses. 247. The eye has six different nerves, each for a different service. 1. The optic nerve. This has nothing to do with the motions or the common sensations of the eye. Its sole office is to transmit impressions from the images formed in the eye to the brain. 2. A nerve of common sensation, by which any irritation in the eye is felt. 3. A nerve which is distributed to the muscles of the eye generally, and to no other parts of this organ. 4. A nerve which goes to one particular muscle, one of the oblique muscles of the eye. It is an involuntary muscle which performs the insensible rolling motions of the eyeball, and is associated with the muscles of expression in the countenance by means of nervous connections. 5. A nerve which goes to another single muscle, which turns the eye outward. 6. A branch of the respiratory nerve, which regulates the motion of the eyelids, and has much to do, therefore, with the expression of the countenance. To this small organ, then, are distributed six different nerves, each having its distinct office, and its separate origin in the brain. How various are the transmissions through these nerves, and how nicely adjusted must all the parts of this complicated apparatus be, that each may perform its office without interference with the rest!

248. I have already alluded incidentally to the fact, that one nerve may be paralyzed, and others distributed to the same parts may be entirely unaffected. Thus the nerve of expression in the face may be paralyzed alone, the face retaining its usual sensibility and its power of performing other motions than those of expression, as mastication, because the nerves of common sensation and of common motion are untouched by the disease. So, too, in the nerves which go out from the spinal marrow, composed of tubuli of motion and sensation mingled together, one set of the tubuli may be affected while the other is not; for in paralysis it is often the case that the sensibility remains while the power of motion is gone, and vice versa. Sir Charles Bell relates an interesting case, in which the paralysis was different on the two sides of the body. A mother was seized with a paralysis, in which there was a loss of muscular power