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THE INTERNAL EAR.

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becomes enabled to dilate the orifice of its corresponding meatus externus to the utmost extent; and by erecting his auriculæ, with their concavities directed forwards, he becomes enabled to collect all the properties of sounds, including of course those which relate to its locality. The same remarks are, with very little exception, applicable to the hare, rabbit, fox, cat, and indeed to every animal which makes extensive use of its auricle*"

The use of the meatus is evident. It is a tube conducting to the tympanum, and must be of importance in regulating the intensity and softening the quality of sounds. The physiology of the membrana tympani will be more appropriately considered after we have explained the anatomical construction of the internal ear.

THE INTERNAL EAR.

Anatomists are accustomed to divide the internal ear into two parts, the tympanum and the labyrinth.

Immediately behind the thin elastic membrane, of which we have already spoken as dividing the external and internal ear, there is an irregular oblong cavity about half an inch in width, called the tympanum. This cavity communicates with the mouth by a small duct, HI (fig. 3), called the eustachian tube. It is bounded externally by the membrana tympani, and internally by an osseous septum separating it from the labyrinth. It was once supposed that deafness would result from the stoppage of the eustachian tube; but from the experiments

*Tod's Anatomy of the Ear, p. 41.

of Dr. Wollaston, which we shall presently have occasion to mention, it is only to sounds of a certain pitch.

The bones of the tympanum form a curious and complicated apparatus. They are usually said to be four in number, and have received names agreeing, it is supposed, with their peculiar forms. SC (fig. 4) is called the malleus, or hammer,

FIG. 4.

which is so placed that its smaller end comes into contact with B P, the incus, or anvil, and V is the stapes, or stirrup, the last two being connected with a small round bone, P, called by anatomists the os orbiculare. There are however, according to the writers on whom we may best depend, but three bones in the human subject, for the os orbiculare is but a process of the incus, though in the horse and other animals it may be distinctly seen as a separate bone. These bones form a chain, and are supposed to communicate the vibrations excited upon the membrane of the tympanum. This, however, is probably not the only use of the curious chain of bones; for when the tympanum is destroyed, and the whole apparatus consequently hangs loose, hearing is not destroyed.

The septum tympani separates the tympanum and the labyrinth, which is an extraordinary system of canals formed in the bony cavity of the skull.

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The labyrinth is divided by anatomists into three partsthe vestibulum, cochlea, and semicircular canals; "and to these may be added an appendix called the aquæductus fallopii." These are represented in fig. 5: a is the vesti

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bulum, which is of an irregular curvilinear triangular figure, in appearance resembling the body of a common padlock, and about the size of a decorticated grain of barley;" bb represents the superior semicircular canals; cc the posterior semicircular canals; d d the inferior semicircular canals; e is the common canal, and ƒ its orifice; g is the situation of the fenestra rotunda; h is the scala tympani; i the scala vestibuli; m the cupola. The whole cavity of the labyrinth is filled with a fluid into which the branches of the auditory

nerve are brought. This fluid is evidently as important to the organ of hearing as the fluids of the eye are to the production of sight; for if by any means the membrane which encloses the labyrinth should be pierced, and the liquid be allowed to escape, deafness is the necessary result.

The membrana tympani, which, as we have already stated, may be considered as the division between the external and internal ear, has, it cannot be doubted, an important office in the organ of which it forms a part. It is intimately connected with the malleus, and through that bone with all the other parts of the internal apparatus. The form, character, position, and texture, of this membrane have led anatomists and philosophers to suppose, that it receives the vibrations excited by a sounding body, and conveys them to the malleus, and the other smaller bones with which it is connected. It is also a remarkable fact, that the membrane is situated a little oblique in reference to the meatus, but less so in man than in those animals to whom an acuteness of hearing is of importance. The object of this is evidently, that as large a surface as possible should be presented to the action of the vibrating substance, and the extent of surface is proportioned to the wants of the animal. None of the organs are so fully developed in man, that animals cannot be found possessing a more perfect organization. The reason of this is evident: animals are governed by their senses, and according to their acuteness is the fitness of the animal to the circumstances in which he is placed. The physical strength and power of flight possessed by the eagle, would be useless expenditures of divine skill if the bird were not also furnished with a quick and penetrating eye. The defenceless hare, pursued by man and animals, would

ORGANIZATION OF THE EAR.

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be in a state inferior to most other creatures, if it had not a quickness of hearing warning it of approaching danger. But man, the master of all, possesses an improveable reason, by which he is regulated more than by his senses. Yet it is worthy remark, that his senses also are not only capable of, but require exercise, and their acuteness increases in proportion to their use. The child is at first quite unable to judge of distances, and, according to the opinions of some writers, perceives everything inverted; but the eye that is thus deceived may be destined to watch and reveal the most occult processes of nature. The ear may be alive to none but the most simple sounds, but may afterwards be tuned to the appreciation of the most scientific and intricate music.

There may also be another reason for the oblique position of the membrana tympani; preventing it "from being injured by the actions which any violent impulses might have produced, and enabling it to respond to the various sounds with great facility."

Allusion has been already made to the office of the small bones called the ossicula auditus; but as we do not feel at liberty to express an opinion of our own upon questions which require for their determination an extensive acquaintance with anatomy, we may be permitted one other quotation, referring the reader to the work from which it is taken. These bones 66 are the most delicate and the most perfect osseous structures in the body, and the least susceptible of disease. From their intimate connexion with the membrana tympani and fenestra ovalis, and with each other; from their zigzag position and their numerous muscles; from the nature of their articulations; and from their being under the influence of a

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