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CONVERSATIONS BETWEEN A MOTHER
AND HER CHILDREN.
[Continued from page 456.]
ON THE EAR.
Mamma. We now come to the third division of the ear, which is called the labyrinth. This is the internal ear, or the proper seat of hearing; all the other or more outward parts being formed to act
Frank. Does it consist only of one cavity ?
Mamma. On the contrary, the labyrinth is divided into the vestibule or middle cavity, the semicircular canals, and the cochlea. It takes its name from the extreme intricacy of its form, and from the inany windings which there are in it.
Emily. Does the middle cavity contain air, as that of the tympanum does ?
Mamma. No, it contains on the contrary an expanded nerve, in contact with which there is an aqueous or watery fluid.
Emily. What is the form of the vestibule ?
Mamma. Its form is oval, and the bore is very small.
Frank. What do you mean by its bore, mamma?
Mamma. The bore is the bollow or perforation of a tube.
Mamma. Of what did the second division of the labyrinth consist ?
Frank. Of the semicircular canals.
Mamma. There are three of these, but their position and leadings are so extremely intricate, even if I attempted a description of them, you would not understand it. We will therefore pass to the third division, which I told you was called—what ?
Emily. The cochlea.
Frank. I think that is Latin for a snail-shell. So I suppose there is something in the form of this part of the ear which resembles that shell ; is there not, mamma?
Mamma. Yes; you have observed in a broken snail-shell, that there is a kind of spiral tube, round which the curling part, or, as they are called, the whirls of the shell, are twisted. Just so it is with the cochlea; the circles at the base being the largest, and getting gradually smaller, till they reach
Frank. And what is the use of the pillar and the whirls ?
Mamma. Both the pillar and the circles which surround it, are of the most exquisite structure, and through them part of a nerve is conveyed and protected. But as I said of the semicircular canals, it would be useless to give you more than a general outline of this section of the ear; its construction is so wonderfully curious and involved.
Frank. Why then, mamma, do you think that this form was chosen?
Mamma. That question is easily answered, my dear ; for the more closely the structure of the ear is examined, the more clearly is it discovered that every part of it is exactly adapted for the conveyance of sound to its nerves and to the brain.
Emily. Mamma, have all the minute divisions of the ear, which you have described, the same provisions of vessels and membranes as the other parts of the body?
Mamma. Yes, they have; the vestibule, the semicircular canals, and the cochlea, have each their periosteum, which is vascular, or supplied with vessels and soft membranes, which contain a fluid.
Emily. Is the base of the ear part of the skall ?
Mamma. Yes, my dear, it is formed on the side of the temporal bone, into wbich the nerves of the ear pass. Now repeat to me the substance of what I have said. How is the ear divided ?
Frank. Into the outer ear, the tympanum, and the labyrinth.
Emily. The outer ear is that wbich we see, which is partly cartilaginous and partly bony.
Jane. The tympanum comes next. It is a hollow, with two holes in it, and it has got a membrane over it, and so have they. And there are four funny little bones in it which are fastened together, and beat upon the membrane like little drum-sticks.
Emily. The labyrinth is the most difficult part of the ear to understand, and is divided into three parts -the vestibule, the semicircular canals, and the cochlea.
Mamma. Very well. Now I will try to explain to you about the faculty of hearing. In the same manner as the eye is the instrument through which impressions relating to the appearances of things
are made upon the brain, so the ear is the organized instrument through which impressions of sound are conveyed to it. Sound is produced by air, which is put in motion by striking against a sonorous body.
Frank. Do you mean, mamma, that air is essential to sound, and that if there was no air there would be no sound?
Mamma. Yes ; this can be proved by experiment. There is a machine called an air-pump, by means of which all the air can be withdrawn from under a glass connected with it, which is called an exhausted receiver. If a bell, which has previously been suspended, is rung when the air is all gone, no sound whatever is heard.
Frank. That does prove it indeed most satisfactorily. How I should like to see an air-pump!
Mamma. You shall see one, my dear, and have the principle upon which it acts clearly explained
Frank. Thank you, mamma.
Mamma. No, water is a liquid--that is, it can be separated into drops, which is not the case with air; but it has several properties in common with water, one particularly, which is, that it always finds its own level.
Emily. I do not quite understand you, mamma.
Mamma. There is a principle in nature called gravitation, which means, that all bodies have a tendency to fall towards the centre of the earth. Water therefore, and air, and all fluids being once displaced, fall again till they have reached the lowest point to
which they can go; that is, till every particle bas sunk to its level: now if you apply this to the air, you will find, that where it is driven out of its level it seeks it again, and this causes vibration or movement in it; and when in these vibrations, it meets with a sonorous body, sound is the result.
Emily. Then are there waves in the air as there are in water ?
Mamma. Yes, and if you remember the effect which is produced when you throw a stone into the water, (which forms circle after circle, till the large outer one is so faint, that you can scarcely see it) you will understand how the air is similarly affected, and that the undulations in it in proportion to their distance from the place where the sonorous body was struck, are fainter and fainter, and at last no sound is heard-sound travels at the rate of 1130 feet in a second-light travels with much greater rapidity : so that if you stand at a considerable distance from the place where a gun is discharged, you will see the flash long before you hear the report of it.
Frank. I suppose that is the reason why we do not hear thunder and see lightning at the same time?
Mamma. It is, and when there is but little space between them, we say that the storm is over-head. The waves, then, of the air are collected by the outer ear; the trumpet-like shape of which conveys the sound to the membrane of the tympanum. Behind this is a cavity containing air, otherwise the membrane would not move backwards and forwards as it does when it is acted upon by external vibrations, Then the chain of bones increases the vibration received on the membrane of the tympanum, and transmits it to the membrane of the foramen ovale. You