LESSON VIII.

SENSATIONS AND SENSORY ORGANS.

1. THE agent by which all the motor organs (except the cilia) described in the preceding Lesson are set at work, is muscular fibre. But, in the living body, muscular fibre is made to contract only by a change which takes place in the motor or efferent nerve, which is distributed to it. This change again is effected only by the activity of the central nervous organ, with which the motor nerve is connected. The central organ is thrown into activity immediately or ultimately, only by the influence of changes which take place in the molecular condition of nerves, called sensory or afferent, which are connected, on the one hand, with the central organ, and, on the other hand, with some other part of the body. Finally, the alteration of the afferent nerve is itself produced only by changes in the condition of the part of the body with which it is connected, and which usually result from external impressions.

2. Thus, the great majority (if not the whole) of the movements of the body and of its parts, are the effect of an influence (technically termed a stimulus or irritation) applied directly, or indirectly, to the ends of afferent nerves, and giving rise to a molecular change, which is propagated along their substance to the central nervous organ with which they are connected. The molecular activity of the afferent

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nerve communicates itself to the central organ, and is then transmitted along the motor nerves, which pass from the central organ to the muscles affected. And, when the disturbance in the molecular condition of the efferent nerves reaches their extremities, it is communicated to the muscular fibres, and causes their particles to take up a new position, so that each fibre shortens and becomes thicker.

3. Such a series of molecular changes as that just described is called a reflex action—the disturbance caused by the irritation being as it were reflected back, along the motor nerves, to the muscles.

A reflex action, strictly so called, takes place without our knowing anything about it, and hundreds of such actions are going on continually in our bodies without our being aware of them. But it very frequently happens that we learn that something is going on, when a stimulus affects our afferent nerves, by having what we call a feeling or sensation. We class sensations along with emotions, and volitions, and thoughts, under the common head of states of consciousness. But what consciousness is, we know not; and how it is that anything so remarkable as a state of consciousness comes about as the result of irritating nervous tissue, is just as unaccountable as the appearance of the Djin when Aladdin rubbed his lamp, or as any other ultimate fact of nature.

4. Sensations are of very various degrees of definiteness. Some arise within ourselves, we know not how or where, and remain vague and undefinable. Such are the sensations of uncomfortableness, or faintness, of fatigue, or of restlessness. We cannot assign any particular place to these sensations, which are very probably the result of affections of the

afferent nerves in general, by the state of the blood, or that of the tissues in which they are distributed. And however real these sensations may be, and however largely they enter into the sum of our pleasures and pains, they tell us absolutely nothing of the external world. They are not only diffuse, but they are also subjective sensations.

5. What is termed the muscular sense is less vaguely localized than the preceding, though its place is still incapable of being very accurately defined. This muscular sensation is the feeling of resistance which arises when any kind of obstacle is opposed to the movement of the body, or of any part of it; and it is something quite different from the feeling of contact, or even of

pressure.

Lay one hand flat on its back upon a table, and rest a disc of cardboard a couple of inches in diameter upon the ends of the outstretched fingers; the only result will be a sensation of contact-the pressure of so light a body being inappreciable. But put a two-pound weight upon the cardboard, and the sensation of contact will be accompanied, or even obscured, by the very different feeling of pressure. Up to this moment the fingers and arm have rested upon the table; but now let the hand be raised from the table, and another new feeling will make its appearance that of resistance to effort. This feeling comes into existence with the exertion of the muscles which raise the arm, and is the consciousness of that exertion given to us by the muscular sense.

Any one who raises or carries a weight, knows well enough that he has this sensation; but he may be greatly puzzled to say where he has it. Nevertheless, the sense itself is very delicate, and enables us to form tolerably accurate judgments of the

relative intensity of resistances. Persons who deal in articles sold by weight, are constantly enabled to form very precise estimates of the weight of such articles by balancing them in their hands; and, in this case, they depend in a great measure upon the muscular sense.

6. In a third group of sensations, each feeling, as it arises, is assigned to a definite part of the body, and is produced by a stimulus applied to that part of the body; but the bodies, or forces, which are competent to act as stimuli, are very various in character. Such are the sensations of touch, taste, and smell, which are restricted to the membranes covering the surface of the body, and lining the mouth and nasal cavities.

And lastly, in a fourth group of sensations, each feeling requires for its production the application of a single kind of stimulus to a very specially modified part of the integument. The latter serves as an intermediator between the physical agent of the sensation and the sensory nerve, which is to convey to the brain the impulse necessary to awake in it that state of consciousness which we call the sensation. Such are the sensations of sight and hearing. The physical agents which can alone awaken these sensations (under natural circumstances) are light and sound. The modified parts of the integument, which alone are competent to intermediate between these agents and the nerves of sight and hearing, are the eye and the ear.

7. In every sensory organ it is necessary to distinguish the terminal expansion of the afferent or sensory nerve, and the structures which intermediate between this expansion and the physical agent which gives rise to the sensation.

And in each group of special sensations there are certain phenomena which arise out of the structure of the organ, and others which result from the operation of the central apparatus of the nervous system upon the materials supplied to it by the

sensory organ.

8. The sense of TOUCH (including that of heat and cold) is possessed, more or less acutely, by all parts of the free surface of the body, and by the walls of the mouth and nasal passages.

Whatever part possesses this sense consists of a membrane (integumentary or mucous) composed of a deep layer made up of fibrous tissue, containing a capillary network and the ultimate terminations of the sensory nerves; and of a superficial layer consisting of epithelial or epidermic cells, among which are neither nerves nor vessels.

Wherever the sense of touch is delicate, the deep layer is not a mere flat expansion, but is raised up into multitudes of small, close-set, conical elevations, which are called papilla. In the skin, the coat of epithelial or epidermic cells does not follow the contour of these papillæ, but dips down between them and forms a tolerably even coat over them. Thus, the points of the papillæ are much nearer the surface than the general plane of the deep layer whence these papillæ proceed.

Loops of vessels enter the papillæ, and the fine ultimate terminations of the sensory nerve-fibres distributed to the skin terminate in them, but in what way has not been thoroughly made out.

In certain cases, the delicate fibrous sheath, or neurilemma, of the nerve which enters the papilla, enlarges in the papilla into an oval swelling, which