a peculiar art in playing these instruments, since no tone whatever is produced, however strong the current of air may be, if they are placed in the open mouth.

The formation of the human larynx is exactly similar to that of the reed-pipe. The tongue is formed by the vocal-chords, two elastic membranes, whose free edges enclose a narrow slit, which, in fig. 62, is artificially imitated by means of sheets of india-rubber. If we blow into the tube which is meant to represent the wind-pipe,

a tone is produced which is not unlike the human voice. The sheets of india-rubber are made to separate, and the slit is thus opened; their elasticity brings them together again, which gives rise to a periodical interruption of the current of air. Such an apparatus is called an artificial Larynx.

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CHAPTER II.

Hearing through the Air and through the Skull-The External Ear and th Auditory Canal-Recognition of the Direction of Sound-Arrangements for the Protection of the Ear.

THE proper sensation of hearing commences at the expansion of the auditory nerve in the labyrinth, whilst, hitherto, the only action has been the conveyance of the sound-waves. The first process in hearing is, therefore, the conveyance of the sound to the fluid of the labyrinth, the waves of which have the power of exciting the expanded nerves. The sound is generally conveyed from the surrounding air to the labyrinth through the auditory canal, the tympanic cavity, and the ear-bones. For, every body which vibrates in the air, throws the air which surrounds it into similar vibrations, which extend outwards from the point of their origin, in the form of a circle, while their intensity decreases with the square of the distance. The vibrations consist of condensations and expansions of the air, which follow one another, like circles. of waves upon the surface of water, as is represented in fig. 63. Sound, however, can pass through solid bodies also, without the intervention of the air, and reach the labyrinth in this manner. If we strike a tuning-fork

N

gently, and hold it before the open mouth, we hear nothing; but as soon as it touches the teeth we hear its tone quite clearly, since it is now conveyed directly to the labyrinth through the skull, without passing through the

tympanic cavity, or the ear-bones, at all. We can also hear the ticking of a watch if we touch it with the teeth, or press it upon the bones of the skull just behind the ear. Moreover, there is a well-known children's game, which depends chiefly upon the conveyance of sound to the ear through bone. A metallic spoon is fastened to a string, which is wound round the first finger, and the finger pressed firmly into the auditory canal. The spoon is then struck against the edge of the table, and the sound produced resembles that of a large bell, for the vibrations. of the spoon are communicated to the labyrinth, through the thread, the finger, and the bones of the skull, with increased intensity.

The communication of sound through bone is of no value to man under ordinary circumstances, since we do not generally bring the head into close contact with the sounding body. In fishes, however, it plays a very important part, for they possess neither external ear, auditory canal, or ear-bones, but only a labyrinth, which is entirely

closed by walls of bone, or is only covered with membrane · on its outer surface. The sound-waves of the water, therefore, are transferred directly to the fluid of the labyrinth through bone. This is possible because the soundwaves of water, which is well known to be incompressible, are similar to those of a solid body, and do not, like those of the air, consist of expansions and condensations. Sound-waves of the air, on the contrary, are transferred to solid bodies and to liquids with great difficulty; and to effect this, ingenious contrivances are required, which are found in their greatest perfection in our ears. Such contrivances are not required by fishes, for they have only to hear in water; indeed, an apparatus filled with air, if placed between the water and the labyrinth of their ear, would considerably diminish their power of hearing. In case of illness the communication of sound through the bones can be of considerable importance to us. If the apparatus for the communication of sound is diseased, and can no longer perform its duties, in spite of deafness, sound can still be communicated by means of bone, which then gives the physician a very important diagnostic sign that the labyrinth and the auditory nerve are in a sound condition.

The regular action with which we have to do is the conveyance of sound through the air, beginning with the reception of the sound-waves by the external ear and the auditory canal, and ending with the transfer of the same to the labyrinthine fluid. In this path there are many ingenious contrivances, which are deeply interesting. Although there is much in this action which is not yet explained with sufficient certainty, still, since the investigations of Helmholtz upon the tympanic cavity,

many discoveries have been made which have opened the way for new enquiries. Of course enigmas still occur, as is generally the case in nature.

A consideration of the curiously coiled folds, furrows, and hollows of the human external ear might lead us to suppose that nature has created all these details for some definite purpose, as well as for the beautiful form which may accompany them, and which nature often puts to some practical purpose. Nevertheless, it would be very difficult, from a scientific point of view, to establish such a supposition, although formerly it had not a few adherents; and we arrive at a very different conclusion, if, as is frequently the practice in physiology, we pass from human anatomy to the animal kingdom, and consider the form of the external ear we meet with there.

The external ear, together with the auditory canal, certainly forms a funnel-shaped ear-trumpet, which, in horses for instance, we find in a form which answers this purpose, as a true funnel with tolerably smooth walls. The marked strengthening of the sound produced by a funnel-shaped ear-trumpet, which can easily be imitated by placing a roll of paper in the ear, is especially remarkable if we close the other ear and then listen to the general rustle in the air; or, still better, if we endeavour to hear the ticking of a watch placed upon the table at a slight distance, with and without the tube. The soundwaves which fall upon the wide opening are reflected inwards by the sides of the tube, and this causes a continual increase in the intensity of the sound-waves, ie., the amount of the condensation and expansion. While this function is fulfilled to a considerable extent by the ears