thus, each of the nerves of special sense-for example, taste, smell, &c.has its own proper centre. The most prominent part of the brain is the cerebrum,1 or, as it is also called, the cerebral hemispheres (because it is divided into two by a deep fissure from front to back), which, on the outside, presents the appearance of a piece of cloth crushed together in irregular folds. On the outside is a coating of gray matter, while in the inside is white matter. Behind and beneath the cerebrum is the cerebellum, or little brain, which is also divided into two hemispheres, and which is specially charged with the regulation of movements. From the brain, and through openings in the skull, proceed a number of nerves to the different organs about the face; and as these organs are for the most part double, the nerves go in pairs; thus, a pair of olfactory,2 or smelling nerves, go to the nose, a pair of optic, or seeing nerves, go to the eyes, and a pair of auditory, or hearing nerves, to the ears. Besides these pairs, there are nerves to the organs of taste, and a number of others both afferent and efferent.

4

From the base of the cerebellum, the spinal cord passes down through the middle of the series of vertebræ forming the spine, sending out nerves to the trunk and limbs; and its principal function is to connect these nerves with the brain, although it has also influence of its own not dependent on the brain. The nerves springing from the spinal cord are all in pairs, arising between each pair of vertebræ, one on each side. The manner in which nerves are supplied to the extremities is through what is called a plexus,5 or network. Several nerves spring from the spinal cord, and proceed together for some distance without meeting; they then divide, and branches from different nerves unite to form new nerves, and these again divide, and interchange fibres; and the nerves thus formed proceed to the extremity, as seen in fig. 58, which represents the nerves of one of the arms.

Besides the system of nerves just described, there is another set of nerves and ganglia in the body. There is a chain of ganglia, connected by nervous cords running

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Fig. 58.

down in front of the spine, which sends off nerves to the viscera and

1 From Greek kara, the head.

3 From Greek optos, seen.

5 From Latin plecto, plexum, to twist or knit.

2 From Latin olfactus, the sense of smell.

4 From Latin audio, to hear.

blood-vessels in the chest, abdomen, and pelvis. This system of ganglia, which seems to regulate, almost independently of the will, the processes of digestion, circulation, and respiration, its influence being little, if at all, dependent on the brain, has therefore been called the nervous system of organic life, or the sympathetic system.

When an impression is made on any of the nerves of the body, it carries this impression to the brain, and causes a sensation. The impressions made on the nerves are not all the same, however; the different kinds of sensations thus caused in the brain are classed into those of Touch, Taste, Smell, Hearing, and Sight.

Touch.

The special seat of the sense of touch is the surface of the skin. The epidermis, it will be remembered, is not sensitive; the parts really sensitive are the papilla, on the outside of the cutis, or true skin. It is to these papillæ that the nerves are distributed on which the impressions are made by the various objects touched, and by which these impressions are conveyed to the brain. They are largest and most numerous on the palms of the hands and the soles of the feet; on the back and outside of the limbs they are much less numerous. They are small elevations, usually conical or club-shaped. Where the sense is acute, however, they have several points, from which they are called compound papillæ. The epidermis is also particularly thin on those parts where the sense of touch is acute, as, for example, on the tips of the fingers and lips. The papillæ are arranged in rows, and it is these rows that cause the fine ridges in the epidermis seen on the palm and fingers.

The simplest idea conveyed to our minds on touching any object is resistance; by the degrees of resistance we meet on touching a body, we judge whether it be hard or soft; and on moving the fingers over the surface, the impression made on the papillæ informs us whether it be smooth or rough. Were it not for our faculty of moving the hands over bodies, indeed, the sense of touch would have been of far less value than it is; for without that faculty we could not have formed our ideas of form and size.

Taste.

The seat of the sense of taste is principally, though not exclusively, on the surface of the tongue, and the arrangement is very similar to that described for Touch. The nerves of taste, like those of touch, are distributed to papillæ, in the outer membrane of the tongue. One remarkable difference in the arrangement of the papillæ of the tongue is that, instead of being concealed in its skin, they project and form the roughness of the tongue, being covered of course by the outer coat. There are

on the tongue both simple and compound papilla, the former very much like those in the skin, the latter of various forms. The sense of taste is very much assisted by that of Smell, as is seen in the case of aromatic substances, the taste of which is much weaker when the nostrils are closed than when they are open. Other substances, called pungent, such as pepper, mustard, &c., when tasted, produce a sharp, pricking sort of sensation, which is also produced on the skin-that is, by the sense of touch-if the same substances be applied to it strongly enough. Touch and taste are thus very closely connected. It may be remarked that the sense of true touch is very acute on the tip of the tongue.

It is always necessary that the tongue be moist in order to taste anything; and it seems, too, to be a condition in substances having taste that they be soluble in water, as bodies that cannot be dissolved in water are quite tasteless.

Smell.

The seat of the sense of smell is in the cavities of the nose into which the nostrils open, and which open behind into the pharynx, or entrance to the windpipe, above the soft palate. The two cavities are separated from each other by a vertical partition, principally composed of cartilage, of which, indeed, the greater part of the framework of the nose is composed. The walls of these cavities are lined with a thick, velvety membrane, over which the olfactory nerves are distributed. It will be remembered that the outer walls are partly formed by the 'turbinated' bones (p. 45), which form three projections on each side, with hollows between. The object of this arrangement is to increase the surface over which the membrane is spread, as it is on this to a great extent that the acuteness of the sense depends. The membrane is kept continually moist by mucus, to which the particles of any substance smelt attach themselves when carried into the nostrils by the air, and thus, coming in contact with the extremities of the olfactory nerves, produce the impression which gives rise to the sensation of smell. It is on the roof of the cavity that the sense is most acute; and the effect of snuffing up the air is to throw the particles on that part, so as to enable us to detect odours that might otherwise escape us; although most people are unconscious of the object of this action.

Hearing.

The ear, the organ of hearing, is more complicated than any of the organs of the senses yet described. It consists of three parts-the external, middle, and internal ears, the last two lying in cavities in the temporal bones. The external ear consists of the fleshy part seen on each side of

the head, which ordinarily receives the name of ear for itself, and of the passage seen entering the head. This passage is completely closed on the inside by a membrane called the membrane of the drum of the ear, and behind this is the middle ear. The latter is a cavity filled with air, received through a tube communicating with the nose, and across which there stretches a chain of curiously shaped little bones. These bones form the communication between the membrane of the drum and one of the membranes covering the two openings into the internal ear, and their use seems to be to communicate the sound-vibrations with greater intensity than if they had been communicated direct to the inner membrane. The internal ear consists of three parts-the vestibule, the semicircular canals, and the cochlea [Latin, 'the shell of a snail']. The vestibule is

Fig. 59.

the middle part V, and communicates with the middle ear by an opening covered with a membrane, to which the end of the bony chain mentioned above is attached; the other two parts are extensions of this cavity (and open into it, as into a hall; hence its name) chiefly for the purpose of giving a greater extent of surface for the membrane in which the fibres of the auditory nerve are distributed. The semicircular canals are three passages, S, in the bone, all lined with the same mem

brane as the vestibule. The cochlea, C, is a spiral canal, very like the inside of a snail-shell, and also lined with the membrane of the vestibule. The passage is divided into two by a partition running through it lengthwise. The two passages thus formed only communicate with each other at the inner end; at the entrance of the spiral, the one passage opens into the vestibule, the other into the middle ear, the latter opening being closed with a membrane. All these cavities are completely filled with a fluid, which is made to vibrate when the vibrations of the air which strike on the membrane of the drum are communicated, through the chain of bones in the middle ear, to the membrane covering the entrance to the vestibule. These vibrations act on the fibres of the auditory nerve distributed over the membrane lining the cavities; and the impressions being communicated to the brain, produce the sensation of sound.

Sight.

The organ of sight is the Eye, which is a spherical body, and composed of several protective coats on the outside, and of a delicate optical instrument in the inside. The outer wall of the eyeball is composed of three coats. The outermost coat, called the sclerotic,1 1, fig. 60, is a strong, tough, fibrous membrane,

the use of which is to protect the parts inside. It does not cover the whole of the eye; the front of the eye, or that part which is visible, is covered with a transparent, horny or cartilaginous plate, called the cornea,2 2, which fits into the edge of the sclerotic, almost exactly in the way the glass of a watch fits into the case. The

Fig. CO.

cornea, as may be seen from the figure, bulges out a little beyond the line of the sclerotic, or is rather more convex. The second coat is the choroid,3 3, a more delicate structure than the last, consisting almost entirely of blood-vessels and nerves. It is of a very dark colour; and it is this dark coat, seen through the narrow opening in the centre of the iris, which causes the round black spot to appear in the centre of the eye. Like the sclerotic, this coat also gives place to another in front of the eye; instead of the choroid, in front is the iris, 6, 6, a coloured membrane, which hangs down behind the cornea. The use of the iris is to act as a curtain, to regulate the amount of light that is to be admitted into the eye; for it is contractile, and can thus diminish the size of the little hole, or pupil,5 7, when the light is too strong, as may be observed in the eye of any one looking at a strong light. The third coat is the retina, 8, so called because it is a complete network of nerve-fibres from the optic nerve, 15, 16, extending all round the back part of the eye, being deficient towards the front.

In the interior of the eye, we come first to the aqueous humour,7 which is a fluid little else than pure water, filling the anterior chamber

1 From Greek skleros, hard, stiff.

3 From Greek chorion, any skin.

2 From Latin cornu, a horn.

4 From Greek iris, a rainbow.

5 From Latin pupillus, a child; because, on looking into it, one sees one's own image reflected exceedingly small.

7 From Latin aqua, water; humour is any fluid or moisture, from Latin humeo, to be moist.