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 th ther from the air. It is covered, as we have seen, or the exterior surface by a continuation of the skir that lines the tube. The middle layer of the membrane of the drum immediately under thi covering is quite transparent, and in a healthy state has no vessels carrying red blood, which car 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 string 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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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 thi ther from the air. It is covered, as we have seen, or 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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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