Page:The New International Encyclopædia 1st ed. v. 06.djvu/668

* EAR. 580 EAR. intensify the iiiipicssiou of llic sound-waves upon the nerve-endings. We now proceed to consider the different functions of the various parts of the ear. The Exteknal Eab. A true auricle exists only in the niamnialia, and in this class it varies from little more than an irregularly shaped car- tilaginous disk, with little or no motion, as in man and the quadruinaiia, to an elongated, funnel-shaped ear-trumpet, movable in all direc- tions by numerous large nuiscles, as in the horse, the rabbit, etc. The mode in which we see it employed in animals in which it is highly devel- oped sulliciently indicates that its main function is to collect the sound-waves that fall upon it, and to direct and concentrate them upon the drum-mend) rane. The .Mh)I)I.e Ear. The vibrations of the tympanic membrane arc transmitted through the ossicles and the membrane covering the fenestra ovalis to the perilymph. The ])ressure in the tym|)anic cavity is kept the same as that of the atmosphere by the Kustacliiau tube. Whenever the drum-membrane is too tightly stretched, by increase or decrease of pressure within the middle ear, it cannot vibrate freely, and hearing is impaired. This often occurs as a result of acute and chronic inflammation of the pharyn., when the swelling of the mucous mem- brane closes the Eustachian tube, producing the so-called 'throat-deafness.' The I,N-reRNi. Ear. Sound is conducted to the labyrinth either by the external and middle ear, as described above ('air conduction'): or through the bones of the head ( 'bone conduc- tion') ; normally air conduction is bettor than bone conduction. In diseases of the middle ear or in obstructions of the external auditory meatus, air conduction is not so good as bone conduction. Advantage is very frequently taken of this fact in the differential diagnosis between diseases of the internal car and those of the mid- dle or of the ext<'rnal ear. To test the air con- duction a vibrating tuning-fork is held close to the ear: to test the bone conduction the handle of the tuning-fork is brought in contact with the teeth, foreliead, nuistoid process, etc., in turn. As stated, it is the base of the stapes that com- municates the vibrations to the perilymph: this would be impossible but for the existence of the fenestra rotundu, which acts as a vent for the wi ^frovAifS H£L fComeM^ ^ JCMA i^snai/^/ (poea ymphJ SCAIA rYMfMN/CPeKILfHPHj ^ C.R f ^CAMAL O^ rffCf^l£-A/£A/OOimPff/ f. HOTUNDA Fig. 6. DIAGRAM TO ILLUSTRATE USE OF FENESTRA ROTUNDA. vibrations of the perilymph and cndolymph ; every time the membrane of the fenestra ovalis bulges in, that of the fenestra rotunda bulges out, and vice versa. The internal ear also performs the most essential function connected with hear- ing: that of perception of auditory impressions by means of the cochlear and vestibular termina- tions of the auditory nerve. Tt remains for us now to consider the function of the semicircular canals. It is now a generally recognized fact that they are not connected with the sense of hearing. They have to do with the maintenance of the ecjuilibrium of the body. This fact was established by Ewald in 18',)2. To maintain equilibrium in the various positions of rest and especially in locomotion, the accurate coordina- tion of a great many nmscles is necessary. The cerebellum is the organ which sends out the nerve-impulses to these muscles; this action is an unconscious one. being purely rellex in char- acter. The atVerent impulses which bring about these relieves are, besides others of secondary importance, the ones conveyed to the cerebellum from the semicircular canals. 'hen these canals are removed in animals, or when they are dis- eased in man, as in .Meni6re's disea.se. there are invariably profound disturbances of equilibrium, the subject being luialde to stand or even to sit, without swaj ing or falling, to say nothing of such complicate<l acts as walking, jumping, etc. The range of hearing, like that of vision, varies remarkably in different pers<ms. .Some persons are insensible to sounds which others can read- ily hear. The ordinary range of liiman hearing includes, according to W(dlaston, more than nine octaves. Technically stated, the normal ear per- ceives vibrations as nuisical notes from 16 vibra- tions per second to about .'i^.oOO. The sensation of sound, like that of light, frequently lasts longer than the exciting cause. Ve have fa- miliar proof of this fact in the noise which re- mains in tlic ears after a long journey in a coach or railway car: and this fact was clearly demon- -strated by Savart. who found, in his experiments on toothed wheels, that the removal of one tooth did not produce any interruption of the sound. DISEASES. The diseases of the ear are divided into dis- eases of the auricle, diseases of the external auditory apparatus, diseases of the middle ear, and diseases of the mastoid process. Oi.sEASKS OF THE AiKici.E. Under this head are included congenital malformati(ms of the auricle, such as deformities, appendages and sui)ermnnerary growths, wounds, injuries, cutane- ous diseases, inllanuuatory processes attacking the cartilage, abscesses, erjsipelas, and tmnors. Diseases of the Kxterxai. .-VrniTORY .ppA- RATis. I'nder this head are included funinclo or boil, cellulitis, impacted wax, foreign bodies in the ear. bony growths, and wounds or other injuries of the drum-membrane. The conditions mentioned in the two foregoing divisions need no more than individual mention, either because their treatment is similar to that of similar conditions elsewhere in the body, or because their titles suggest the remedies appro- priate. Diseases of the Middle Ear. (1) Tnhnl enlarrh is a congestive condition of the Eusta- chian tube, leading from the throat to the middla ear. The result of this affection is closure of the tube, with deafness and constant noises heard, with a sense of fullness in the head. Infla- tion of the tube is the treatment, with possibly subsequent dilatation and treatment with medi- cated vapors. (2) Acntc catarrhal inflammation of the middle ear is an affection of the superfi- cial structures that line the middle ear, with the production of sero-pus in such quantity that often the ear-drum bulges and may even burst. Inci- sion is necessary' to prevent bursting and to secure drainage. The malady may complicate measle.s or catarrh of nose and throat, or nuiv be caused