smooth; but it will not taste it at all; in fact, nothing can be tasted, unless it can be dissolved in water, i.e., in the saliva of the mouth. So, if you put a lump of ice into your mouth, your tongue will feel by the touch that it is cold, and by taste that it is water and not milk. The object of this sense is twofold, first, to decide what is good for food; secondly, how much saliva is required to moisten it; so the taste tells us that a certain article is nasty, that we may either refuse it altogether, or, as in the case of medicine, take it only in small quantities; while, on the contrary, it pronounces others sweet, requiring a large quantity of saliva to commence their digestion, which, as we have seen with these substances, takes place in the mouth, not in the stomach, and immediately our "mouths water," as we express it.

SMELL is exercised by the nose, which is divided into two chambers, called nostrils, by a thin gristly partition, on which there is spread out a very fine net-work of nerves, the branches of one great nerve (the olfactory)

Fig. 26. Vertical Longitudinal Section of the Nasal Cavity. Showing Olfactory Lobe and distribution of the Olfactory Filaments, and the Fifth (Trigeminel) Nerve on the Right wall of the Nose.

which runs straight to the brain. Any fine grains of matter, like dust, striking against this net-work set it vibrating, or all of a tremor, just like the strings of a piano when they are struck, or of a fiddle when the fiddle-bow is drawn across them. Now you know the feeling of vibration from the stinging of your hands sometimes when you hit a cricket ball with your bat; it

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seems to run all up your arm. When fine grains of dust hit these delicate nerves in the nose, they make it sting, and you feel a prickly stinging sensation right up into your head. Try with a bottle of smelling salts or some mustard. That is the vibration running along the olfactory nerve up into the brain; and practice teaches us to distinguish between the various kinds of vibrations, so that in time we learn to know what sort of things produce each of them, and without looking, we can tell whether we are smelling mustard or coffee. But we are not always conscious that there are any fine grains striking against our nostrils, as when we smell a flower. We must not wonder at that, as they are often too small to be seen; yet if you look at a ray of light shining into a room, you will see thousands of little specks of dust floating in the air, that you never see at other times.

The use of smell is partly to guide us in the choice of food, and partly to keep us from poisoning our blood by breathing foul air. Were it not for this we should stand without discomfort over a cess-pool, and keep our windows shut, when all the air in the room was poisonous, and so be inhaling air that was killing us or giving us fevers.

SIGHT is the effect of another vibration acting upon the optic nerve, and is caused by the effect of light. The optic nerve is a branch of the brain, running off in two arms, one to each eye. It pierces through the bone at the back of the socket of the eye, near the nose, and then is distributed over the skin of that socket in a sort of net-work (like that of the olfactory nerve), and this net-work is called the retina, which is like a mirror, placed behind the eye, receiving the objects upon it that are in front of the eye. The eye itself is only a little machine, like that box which is used by photographers to take views or likenesses. If you have your likeness taken, you stand in front of that box, opposite to a round hole, which is covered at first with a lid. When you are properly fixed, the photographer takes off the lid, and the light shining all round you, leaves your shadow on the glasses in that hole.

These glasses reflect your image through the box to a plate of glass behind, and, after a few minutes or seconds, your image is printed on the glass. The retina of the eye is like that plate of glass, and your eye is the box with the round hole and a glass (lens we call it), fixed

Showing the formation of inverted optical images on the
Retina at the back of the Eye.

Fig. 27.

behind the hole. But the retina is much more sensitive than a plate of glass, so it receives the image in front instantly, and this reception of the image on it, sets the roots of the optic nerve quivering and vibrating, and so they telegraph to the brain that something is in front of the eye; and by practice we learn to know one object from another. But our eyes are not glass, they are water, and you can tell for yourselves, that water will do as well for this purpose as glass, by looking at yourself in a basin of clear water, or looking into a pond or pool, in which you can see the trees, houses, or other objects around reflected, just as if it were a looking-glass. These objects are really impressed on the bottom of the pool (as they are on the retina of the eye), and not on the surface of the water.

HEARING is the sense exercised by our two ears, and is also caused by vibrations. I dare say you have thrown a stone into a pond, and you have noticed that

where it falls into the water it makes a splash, and as soon as the splash has ceased, you have seen a ring all round the hole, where it went in, and this ring has gone on widening, larger and larger, till it has got to the edge of the pond. The same thing happens in the air (which is a liquid like water). An object strikes the air and causes a concussion, which sets the air vibrating, just as a stone thrown into the water disturbs the water. The air, too, vibrates in rings, spreading wider and wider, till they are lost to our ears as the rings of water are to our eyes. When any of these rings come within the range of our ears, part of the wave of sound goes into the ears, up the external meatus, pressing in a piece of fine skin, the

membrane Tympani, fitted at the end of it, like the parchment over the ends of a drum. Fixed to the inner surface of this drum-head is a little bone (called the hammer), working on a hinge, with another little bone (the anvil), which with a third (the stapes, or stirrup), orm a sort of

crank which presses inwards another piece of skin, covering a chamber, called the vestibule, to which are attached the semicircular canals, containing water and sand. When the wave of sound pushes in the skin of the drum, it works the crank, which pushes in the inner skin, and sets all the water in the bags moving, and shaking all the grains of sand. These little grains knock against the fine ends of the auditory nerve, (just as the grains of matter do on the olfactory nerve), and make them

due to

Side View of Human Brain, showing Cerebral Lobes and Cranial Nerves (of Right Hemisphere), Cerebellum, Medulla Oblongata, and Corpus Callosum.

The bserver is supposed to be looking at the right side of the great Longitudinal Fissure, and the cut portion of the Corpus Callosum.

Fig. 29.

vibrate; so the sound is carried to the brain. A very curious little instrument called the cochlea, or shell, like a very small piano, with different keys, is attached to these water-bags, and different sounds touch different