may, by practice, become more acute when another has been lost. The organs of special sense likewise enjoy common sensation, but an additional nerve is provided for this. Thus, the eye has the optic nerve for sight, and part of the fifth nerve for common feeling, and either function may be lost, by disease, while the other remains. 2. A conductor of the impression from the organ to the brain. The sensory nerves do not differ in structure from the motor nerves, being simply conductors, the function being determined by the organs and centres with which they are connected. 3. A centre to receive the impression from the nerves. Most of the sensory nerves enter the spinal cord, and the impression is forwarded to the brain, where all sensation is in reality felt, although we refer it to the part from which the nerve came. For instance, if the ulnar nerve, which supplies the little finger, be divided at the elbow, the finger loses all feeling, so that it may be cut or burned without pain. But, if the upper end of the divided nerve be touched, we feel the pain as if in the little finger. The impression has gone up to that part of the brain which has always been accustomed to receive impressions from that finger. Hence persons who have lost an arm or leg not unfrequently complain, long after, of pain as if in the lost parts.

The Senses are the inlets of knowledge, whether for the welfare of the body or the education of the mind; and the exercise of each is made, in addition, a source of pleasure. They are five in number. 1. Touch or common sensation, with the skin for its organ, but more or less disseminated through the body. 2. Hearing. Its organ, the ear, is a complex apparatus, constructed for receiving and transmitting sonorous vibrations. 3. Sight. 4. Smell, the organ of which is the upper part of the interior of the nose, with the olfactory nerve, appropriately placed to guard the entrance to the lungs. And, 5. Taste, guarding the entrance to the stomach, one of the endowments of the tongue, which, like the ear, eye, and nose, is also freely supplied with common or tactile sensibility.

1. The SKIN is a complex structure, serving variously the purposes of protection, touch, and secretion. It presents two layers, the outer or scarf-skin, or epidermis, and the deeper or true skin, or cutis vera. The epidermis1 serves to protect the sensitive true skin. When separated by a blister, the true skin is exposed, and touch

1 Epi, upon; derma, the skin.

becomes painful. The deeper cells of the epidermis contain the pigment,1 or colouring matter, which is present in the white as well as in the coloured skin, the difference being one of quantity only, and having reference mainly to the influence of light. The colour of a dark skin is thrown off by a blister, and is reproduced with the new epidermis. Markings of the skin made by the process of tattooing, or puncturing with needles dipped in colours, are permanent, because they are stainings of the true skin. The cutis2 vera is tough, vascular, and sensitive. When touch is highly developed, numerous little papillæ stand out from the surface of the true skin, and are the especial organs of touch. On the palm and fingers they are ranged in rows, and occasion the surface ridges which the epidermis presents. On the last division of the fingers, these ridges may be seen to run more or less round a centre near the prominent part of the pulpy cushion there formed for touching with. The leather of commerce is the cutis vera of various animals, changed and preserved by the process of tanning. The glands of the skin are of two kinds, the sebaceous glands3 which secrete an oily or unctuous substance to lubricate the surface; and the sudoriferous or sweat glands. The latter resemble a small soft tube coiled up into a ball, less than a small pin's head, and lying on the deep surface of the skin. The tube, leaving the little ball, like a duct, pierces through the skin, winding spirally through the epidermis, and opens on the surface by one of the little pores which may be seen on the ridges of the palm, full of moisture, when the hand is warm. These glands are found over the whole body, and when added together form a very extensive excreting surface, by which a large amount of fluid is daily thrown off, equal to about half that which passes off by the kidneys. Hence the importance of cleanliness to health; of frequent changing and washing of the underclothing, and of regular ablution of the skin. Hence, in short, the importance of soap and water; and the importance also of our bearing in mind that the face and hands are not the only parts which require the use of these important means of cleanliness and consequent sources of health. This law of health is greatly neglected, to the injury of other organs on which the additional work is necessarily thrown, and to the loss not only of the bodily advantages of a clean and healthy skin, but of its moral influence, an influence which gave rise to the old saying that "cleanliness is next to godliness." 2. The EYE consists essentially of an expansion of the optic (or i Pingo, to paint.

2 Cutis, the skin.

Sebum, suet or lard. 4 Sudor, sweat.

seeing) nerve, the retina, for the purpose of receiving luminous impressions; and of a series of humours to act as lenses, arranging the rays in such a way that they may fall correctly on the retina. Various other structures are necessary to carry out these objects and complete the eyeball as a living optical instrument. Next come a series of

The Iris is seen to intercept the outer rays of light, while the others meet

in a focus on the Retina.

muscles and nerves to move the eyeballs; and, lastly, the defensive and moistening apparatus provided in the eyelids and lachrymal

organs.

The Cornea, the clear front of the eye, forms at once a wall to enclose the humours and a window to admit light. Behind it, like a screen, is the bright and variously coloured Iris, with its dark round central aperture, the Pupil, enlarging and contracting by the action of the iris, so as to regulate the amount of light admitted into the deep chamber of the eye.

The middle and hinder part of the eye consists of three coats or layers. 1. Externally, the white Sclerotica, tough and strong like white leather, forming, with the cornea which it joins, a general protecting case. 2. The Choroid, composed mainly of blood-vessels, for the nourishment of the eye, and stained of a deep brown colour by the Pigment, or animal paint, which lines and darkens the interior of the eye behind the pupil. 3. The Retina,

1 Compare Lessons on Light in this volume, pp. 51-60.

the nervous or sensitive coat, on which impressions are formed and thence transmitted by the optic nerve to the brain.

The shell thus formed is distended by three Humours, or refracting media, the office of which is to arrange the rays of light in a focus on the retina, so that an impression-picture of external objects may be formed on it.1 The Aqueous humour occupies the front, before and behind the iris, and is quite fluid, so as to permit of the motions of the iris. Behind is the Vitreous humour, occupying much the greatest space of the eye, and of jelly consistence. Between these is placed the Crystalline Lens. This is a double convex lens, like a magnifying glass, capable of being moved or altered, so as to change the focus of the eye, and adapt it for the vision of near or distant objects. Thus the eye performs what no human instrument can, the double office of a microscope and a telescope. This lens is placed immediately behind the iris, partially sunk into the vitreous humour, and is contained within a capsule, like a nut within a close-fitting shell; and, like the other humours of the eye, it is perfectly transparent.1

FIG. 8.

1. Superior Rectus.

2. Inferior Rectus.

3. External Rectus.

5. Superior oblique, and P its Pulley.

The eye-ball is moved by six Muscles. The Recti, or straight muscles, four in number, proceed from the apex of the socket behind the eye, and passing forwards, like pieces of red ribbon laid against the eye, are fixed by short tendons to the white coat, about a quarter or third of an inch behind the cornea. They are-one superior, one inferior, one internal, and one external; each revolving the eye in its own direction, or in intermediate directions, when two neighbouring muscles act together. The other pair They embrace the eye somewhat

6. Inferior oblique. The dark parts of the muscles are fleshy; the pale parts their tendons.

of muscles are termed oblique.

1 The eye of a sheep, or other quadruped, may be used to illustrate the above.

transversely, passing from the inside, one above, the other below; and their action, long misunderstood, is to roll the eye on its axis, when the head inclines to either shoulder, so as to keep the eye fixed on the object viewed. The superior oblique arises behind with the recti, but its tendon is reflected through a little pulley, which gives it the transverse action. Inward squinting of the eye is owing to the contraction of the internal rectus muscle, and, when established, can be removed only by division of the muscle; which, when efficiently performed, is not only simple and safe, but very successful in removing the distortion, as well as in restoring the vision of the eye.

One eyelid, the

The eyelids are moved by two muscles; one, from within the orbit or socket, raises the upper eyelid ; the other, lying flat on and around the eyelids, is used in closing the eye. upper, is made much larger than the other, as in this way one opening muscle is sufficient, while the eye is at the same time more completely washed and covered, as the line of meeting of the lids is below the cornea.

The eyelashes serve to protect the eye from dust, and to shade it from the light. Those of the upper lid are therefore longest and most numerous, and for a like reason the eyebrow is above the eye, as the sunlight comes from above. The eyelashes of the two lids, by curving first towards, and then away from each other, are enabled to meet before the eyelids meet, and thus exclude dust without excluding light. Their greater thickness at the middle, like the porcupine quill, gives them strength at the curve; and having attained a certain length, their growth ceases, as further growth would be extremely inconvenient. The growing and placing of an eyelash is a matter of no small importance, for the eye may be destroyed by a few growing in upon it. The lachrymal fluid is secreted by a small almond-shaped body, the lachrymal1 gland, which lies at the upper and outer side of the eye. About eight or ten fine ducts, or pipes, bring down the fluid, and shed it behind the outer half of the upper lid. It now flows over the surface of the eye, or is washed over it by the constant winking motion of the lid, which at the same time wipes off impurities. It would not do to allow this fluid to evaporate, as it would leave a residuum on the eye. It is therefore removed to the nose. Near the inner end of each eyelid may be seen, on

1 Lachryma, a tear.

G