THE EUSTACHIAN TUBE.

Musicians are well aware that a violin, or a common drum, will not sound well, without a hole in it, to cause the sound produced to come with more force by a communication with the air without. The sound, indeed, would otherwise be imprisoned in the instrument, and would be heard muffled and confined by the surrounding wood. The same would be the case in the ear. The sound transmitted from the drum to the bony springs, and by them to the parts of the labyrinth, would be smothered in its passage almost as soon as it had passed the drum. To prevent this there is a contrivance exactly like the hole in the common drum and in the violin.

Behind the drum of the ear, a tube opens for this purpose and runs by the side of the labyrinth, widening as it goes, till it ends in a trumpet-like opening behind the curtain which separates the nostrils and the mouth. It is called the Eustachian tube from its describer Eustachius. It was fancifully supposed in former times that this tube was a drain for the corrupt humours of the ear, as the nose was for those of the brain. But were this true, (as it is only a fancy) the throat is certainly not the most eligible part to make a common sewer for corruption. Immediately within the inner termination, indeed, are numerous wrinkles

and cells which give out mucus similar to the mu cus of the nose; the skin which lines it being a continuation of the skin of the nostrils.

The use of this tube, as we have now explained it, and as we formerly alluded to it, page 17 above, is perhaps more correctly understood than any other part of the ear. It is so indispensable indeed to perfect hearing, that when it is in any way obstructed, there always follows some degree of deafness. When it is opened also, by opening the mouth, we hear better than when the mouth is shut. M. Itard controverts the received opinion of sound being transmitted through the tube; for though we hear the tick of a watch better by putting it into the mouth in contact with the teeth, yet by putting it back on the tongue and shutting the mouth, it cannot be heard at all.

The Eustachian tube, however, is but little exposed at its inner termination to obstruction from extraneous substances. It is too high placed, and too well defended by the curtain between the mouth and nose, to be liable to accidents from particles of food or drink getting into its mouth. The chief cause of obstruction here is inflammation from colds or other causes. Most people, indeed, have experienced some time in their lives a temporary deafness, or imperfect hearing, from cold affecting the nostrils or the throat, and

THE TYMPANUM, OR DRUM OF THE EAR. The drum of the ear consists of a membrane, or thin parchment-like skin, stretched obliquely across the bottom of the tube, for the reception and reverberation of the sounds that are propagated thither from the air. It is covered, as we have seen, on the exterior surface by a continuation of the skin that lines the tube. The middle layer of the membrane of the drum immediately under this covering is quite transparent, and in a healthy state has no vessels carrying red blood, which can be discovered. When this is inflamed, however, it becomes quite red with blood-vessels, as well as the exterior and interior layers which cover it. It is remarkable that the whole membrane is braced like the head of a drum; but, in place of the strings of the drum, the membrane of the ear is stretched and kept tight by the threads of a muscle. So at least it is conjectured by Sir Everard Home from analogy; for these threads, it is to be remarked. have as yet been only discovered in large animals, such as the elephant. When any foreign substance touches the drum, exquisite pain is produced, in consequence of its great sensibility. The drum, it is also to be observed, is not quite plain, but a little depressed inwards, as if kept down in the middle by a weight.

CHAIN OF BONES.

Behind the drum is placed a chain of small bones, or springs, so arranged as to hinge upon one another, and when one moves all the others follow, in consequence of their connexion with small muscles, the utility of which will appear as we proceed. There are four of these bones, which from their several shapes have been called the hammer, the anvil, the lentil, and the stirrup. The first, which is shaped like a hammer, is in contact with the middle of the drum, and when a sound strikes upon the drum, it must put this bone in motion; that is, the sound causes the muscles which are attached to the bone, to contract and pull it; while this movement consequently re-acts upon the drum. When the motion is thus originated, it is communicated in succession to the other three bones of the chain, the last of which is the one that has the form of a stirrup, with a solid base, and this plays, like the key of a flute upon a hole, or opening, that leads to several channels or passages farther inward. The internal ear has several of these passages, which are much in the form of wind instruments, of the convoluted kind, being full of turns and windings, like a French horn, and so intricate as to have obtained the name of the labyrinth.

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which we cannot pass in the explanation of the works of nature. We go as far as we can, but we are compelled to stop somewhere.

1. ANATOMY OF THE EAR.

The ear is usually described to consist of two divisions, the outward and the inward, which are separated by what is well known by the name of the drum; or we may consider it as divided into the external, the middle, and the internal ear—a division which we shall follow in our description.

THE EXTERNAL SHELL OF THE EAR.

In different individuals the size of the external cartilage or shell, is very different. The outer surface of it, which in a well-formed ear projects a little forward, presents on its anterior surface five eminences and three cavities. The whole is spread out into a sort of funnel, well adapted from its windings for collecting the waves or pulses of sound afloat in the air around. The substance is also carefully fitted for its office, being very sensible and composed of a firm though elastic texture of gristle or cartilage, and not of unyielding bone, nor of soft fleshy muscle, though there are small muscles which stretch or relax it as occasion may require. The skin which covers it is thin and dry, and is attached to the gristle below by a strong tissue,