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it contracts or expands upon the sudden announcement of good or bad news—its muscular walls being strained too far in the upward or downward direction to enable them to return-and one of the purposes which this property of the heart is probably designed to subserve is to put a check upon the passions through the alarming physical sensations they excite.

The brain, again, is enclosed in a bony case. All our bodily sensations are dependent upon the nerves, but even the nerves do not give rise to feeling unless they are in connexion with the brain. The nervous chord which, in familiar language, is called the spinal marrow, is the channel by which this communication is kept up as to the major part of them, and when a section of what may

be termed the great trunk-road for the conveyance of our sensations is diseased, and by the breach in its continuity the nerves below the disordered part can no longer send their accustomed intelligence to the brain, the portion of the body which thus becomes isolated may be burned or hacked, and no more pain will result than if it belonged to a dead carcass instead of to a living man. The brain, therefore, in subordination to the mind, is the physical centre of all sensation. Yet, strange to say, it is itself insensible to the wounds which are torture to the skin, and which wounds the brain alone enables us to feel. • It is as insensible,' says Sir Charles Bell, as the leather of our shoe, and a piece may be cut off without interrupting the patient in the sentence that he is uttering.' Because the bone which envelopes it is its protection against injuries from without, it has no perception of them when directed against its own fabric, though it is at the same time the sole source of the pain which those injuries inflict upon the other portions of the system. But the skull is no defence against the effects of intemperance, or a vitiated atmosphere, or too great mental toil. To these consequently the same brain, which has been created insensible to the cut of the knife, is rendered fully alive, and giddiness, head-aches, and apoplectic oppression give ample notice to us to stop the evil, unless we are prepared to pay the penalty.

Since neither pain can be felt, nor any other sense can be exercised except through the medium of the nerves, it is to these that we must trace the diversified impressions of which the body is susceptible. It is here that Sir Charles Bell made the beautiful discovery which entitles him to be ranked among the greatest physiologists the world has produced. Pairs of nerves are given off from the spinal cord at short intervals along its entire length. Ramifying throughout the body, they are the medium of communication between the various textures and the spine, and, by means of the spine, with the brain. Each nerve has two roots which issue

separately

rately from the side of the spinal cord, but almost immediately coalesce and run together like a single cylinder. Sir Charles Bell detected the leading fact, which has thrown such a flood of light upon the nervous system, that one of these roots consisted exclusively of sensitive fibres, and its fellow entirely of fibres of motion. Irritate the root which emerges nearest to the back of the cord, and the suffering is intense. Irritate the root which comes out towards the front of the cord, and no pain whatever is felt, but irrepressible muscular movements are provoked. Again, divide the first, which is called the posterior root, and the sensibility of the parts which it supplies is destroyed, while the power of motion remains complete. Divide the second or anterior root, and there is an end to motion in the parts to which its fibres lead, while the sensation continues as acute as ever. The two sorts of fibres which run together for a large portion of their length in a single cord, and which are apparently identical in structure, have yet offices as distinct as seeing and hearing, and which can be no more interchanged than we can hear with the eye or see with the ear. The same nerve, for anything we can discover to the contrary, might have had the double endowment of giving rise to both feeling and motion, just as the nerve of taste appears to be also a nerve of common sensation, but this would have confounded the entire scheme for the regulation of pain. The muscles which are constructed for producing movement must be pervaded by motor nerves.

If these bad been as instrumental in exciting feeling as in causing the contractions by which we sit down, stand up, run, walk, raise weights, and strike blows, the interior textures would have been as sensitive as the skin, and sitting down, standing up, running, and walking would have been operations as painful as a disease. In the marvellous plan of Providence similar fibres have been invested with separate functions; and the hidden muscles being plentifully supplied with nerves of motion, and sparingly furnished with the nerves of sensation which confer such exquisite properties upon our outer integuments, each organ fulfils its own end without detriment to the system.

Notwithstanding the subdued sensibility of the muscles, they nevertheless are possessed of a property which has been termed by Sir Charles Bell the muscular sense,

-a sense which is absolutely essential to the sustained performance of many of the commonest actions of life. If, he says, we shut our eyes, we can still tell the position of our limbs--whether the arm, for instance, is beld out, or whether it hangs loose by our side. By what means is the mind cognisant of this circumstance, since we neither touch nor see anything? Mainly by a consciousness proceeding from

the

the muscles themselves, which informs us of their state, and tells us where they are and what they are about when there is no second channel through which the knowledge can be fully attained. The cases in which the faculty is destroyed best show its use.

Sir Charles Bell attended a woman who had lost the muscular power of one arm, but retained it in the other. Though the muscular power, however, remained, the muscular sensibility was extinct, and the result was that when she used the serviceable arm to hold her infant to her bosom, it only did its duty while her eyes were kept fixed upon it. The moment any object withdrew her attention her arm gradually relaxed, and the child was in danger of falling. In the same way we have seen a paralytic who could raise his glass to his lips as long as he continued to gaze upon it, but if he looked off it for a second it slipped through his hands. In these instances there is no longer a muscular sense to acquaint us with what the muscles are doing, and to regulate their exercise. The necessary knowledge can then be obtained through the vision alone, and directly this source of information fails us also, the muscles speedily cease to exert themselves, just as if there were no glass or infant to sustain. The blind man in such a case would have no use from his arms at all, and in those who can see, how imperfectly does the visual supply the place of the muscular sense! how beautiful is the adaptation which, in withholding from particular textures the sensitiveness which occasions needless pain, yet confers upon them a nicety of perception which reveals to the mind every change in their position, and their precise adjustment when they are at rest!

The principle is apparent in all the special nerves of sense. They have a sensibility of the kind which the particular organ requires, but they are dead to every feeling besides. Unless the same nervous trunk contains fibres differing in function, the nerve of taste, as we have already intimated, is equally a nerve of cominon sensibility. But this is no exception to the rule which ordains that the sensibility shall be limited to what its purpose demands. That we may not introduce substances into our mouths so hot or so cold as to destroy the parts with which they come in contact, it was necessary that the tongue should be a judge of temperature; and that we might the better manage our food in mastication, it was needful that it should have a perception of the surfaces of objects. These properties must be exercised in conjunction with the taste; and whether both are effected through a single nerve, or whether the nerves of touch and taste are distinct fibres blended into one cord, makes no difference in the con

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trivance. Both sensations exist at the same point, because both are required there for the pleasure and welfare of man.

The olfactory nerve is neither capable of producing motion or experiencing ordinary pain. Though there are nerves of common sensibility in its neighbourhood, which are roused by irritating applications, such as snuff, and by the various causes which affect the skin, the nerve of smell perceives odours, and odours alone. The nerve of hearing, in like manner, can only hear, and the nerve of vision, with the exception of being concerned in certain muscular movements, can only see. A substance

may

be designed to address itself to more senses than one, as the food which is pleasant to the palate may be no less grateful to the nose, but it cannot on that account be smelt by the first or tasted by the second. Each sense is kept to its own sphere; and though the same object should put them all into action at the same moment, they would all of them return a different response, and all be true to themselves. Nay, they will answer to a stimulus which has no resemblance to that for which they were primarily contrived; but however much the stimulus may vary, the sense will not depart from its regular function. Thus, if a small current of air is directed to the tongue, it occasions a taste like saltpetre; if the nerve of hearing is irritated, it gives the sensation of sound ; if the retina, which is the expanded nerve of vision, is pricked, as in the operation of couching for the cataract, it gives the sensation of a spark. An officer,' says Sir Charles Bell, who was shot through the bones of the face, felt as if there had been a flash of lightning, accompanied with a sound like the shutting of the door of St. Paul's. A blow from a fist will produce similar effects in a minor degree. An accumulation of blood in the capillary vessels of the several nerves will set the whole of the senses to work. This one cause,' says Dr. Kirkes in his excellent · Handbook of Physiology,'' begets in the retina, while the eyes are closed, the sensation of light; in the auditory nerve, the sensation of humming and ringing ; in the olfactory nerves,

the sense of odours; and in the nerves of feeling, the sensation of pain.' No wonder that sounds are often heard when there is no noise, and luminous appearances seen when there is no light, since the excitement of the nerves by the prick of a needle or the congestion of disease is ample for the purpose. The simple pressure of the finger upon the eyeball will evoke all the colours of the rainbow. In the midst of this insensibility of the nerves of special sense to every sensation except that which is fitted to the function for which each was intended, they have yet a protective pain of their own, which is no less efficient for its

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end than that of the skin. The nose is impatient of bad smells, and impels us to shun their noxious influence. A single organ of limited extent serves in this way to guard the entire body from one class of evils. How intolerable would have been the annoyance, how useless, and perhaps how fatal, if the property had been spread over the whole of the outer integuments, and we had been as sensible of stenches at every pore as of cuts and of burns ! The optic nerve, which is unconscious of other kinds of injuries, is utterly intolerant of a too dazzling light. Placed at some distance beyond the surface, the bones of the skull and the sensitive coat of the eye are its security against wounds; but unless the orb of day was to be the plague instead of the blessing of man, the skin which is our protection against so many dangers could not have been made incapable of being turned to the full blaze of a meridian sun, while with all its obtuseness to laceration, the optic nerve is alive to evils from which there is no other defence, and is its own guardian against an excess of light.

But we have not yet done with the visual organ. The more the instances are multiplied the more we are impressed with the beneficence of the arrangement, and it is especially conspicuous in what Sir Charles Bell relates of the peculiar nature of the sensibility which protects the coat of the eye. The oculist,' he

• , says, 'has observed that if it be touched as lightly as by a feather the muscles are thrown into uncontrollable spasms; but if the point of the finger be passed somewhat rudely between the eyelids so as to press directly upon the eye itself, he can hold the eye steady for his intended operation, and produce hardly any sensation, certainly no suffering. This is one of the little secrets of the art; and still the wonder grows that he can do such things without inflicting pain, when daily experience makes us sensible that even a grain of sand produces the greatest torture. The question is, why the membranes should be keenly alive to the lighter touch, and comparatively indifferent to the rougher; and admirable is the answer which Sir Charles Bell has supplied. Numberless small particles float about in the air, and rest upon the eye, or lodge under the eyelid. Owing to the extreme susceptibility of the surface, these foreign bodies are the agents of their own removal, for they stimulate the flow of tears and the winking of the lid, which together wash the ball from every impurity. The action is proceeding during all our waking hours; and here, as in other instances, the contrivance and its purpose are only revealed to us through the deplorable consequences which ensue from the extinction of the power. The nerve of the coat of the eye is sometimes injured, and is no longer sensitive to

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