PHYSIOLOGY OF THE INTERNAL EAR. 195 



and that without, so as to prevent the inordinate tension of the mem- 

 brane, that would be produced by too great or too little pressure on 

 either side ; the effect of which would be impaired hearing. 



By closing the nose and mouth, and forcing air from the lungs, we 

 can feel a sensation of fulness in the ear, produced by the pressure of 

 the air against the internal surface of the membrana tympani; and 

 they, who have the membrane perforated, can send tobacco smoke 

 copiously out of the external ear. 



Besides this necessary function, the Eustachian tube has been sup- 

 posed to possess another, that of serving as a second meatus auditorius, 

 by permitting sonorous vibrations to enter the pharyngeal extremity, 

 and, in this way, attain the middle ear. A simple experiment, first 

 described by M. Pdrolle, 1 exhibits the fallacy of this notion. If we 

 carry a watch far back into the mouth, taking care not to touch the 

 teeth, little or no sound will be heard ; but if we draw the watch for- 

 ward, so as to touch the teeth, the ticking becomes distinctly audible. 

 If the pharyngeal extremity acted as a second meatus, the sound ought 

 to be heard better when the watch is placed nearer to it; but such is 

 not the case. On the contrary, it is not until the sonorous body is put 

 in contact with the teeth, that the sound is appreciated. This is effected 

 by the vibrations of the watch being conveyed along the bony parietes 

 until they reach the auditory nerve. Again ; if the meatus auditorius 

 externus be completely closed, we cannot hear the voice of one who 

 speaks into the mouth; and can hear but imperfectly our own. The 

 fact of our gaping, when desirous of hearing accurately, has partly led 

 to the belief, that the tube acts as a second meatus. It has been pro- 

 perly remarked, however, that this may be merely an act of expression; 

 and, also, that the meatus auditorius is rendered more open, when we 

 depress the lower jaw, than when it is raised, as may be perceived by 

 inserting the little finger into the meatus, when the jaw is in either 

 situation. 



In addition to these functions, it is probable, that the tube acts as a 

 diverticulum for the air in the cavity of the tympanum, when the mem- 

 brane is agitated by too powerful sounds. The closure of the tube is 

 the cause of that form of deafness, which is relieved by injection of 

 air or other fluids into it a fact, the knowledge of which has been the 

 foundation of much empiricism. It likewise conveys into the pharynx 

 the mucus secreted by the lining membrane of the tympanum, probably 

 by means of the vibratile cilia on its mucous surface. 



Internal Ear. In the various ways mentioned, the vibrations of a 

 sonorous body reach the internal ear. The membranes of the foramen 

 ovale and foramen rotundum resemble the membrana tympani in their 

 physical characteristics; and when thrown into vibrations communicate 

 the impression to the liquor of Cotunnius contained in the cavities of 

 the internal ear. By this medium, the vibrations are conducted to the 

 auditory nerve, which conveys the impression to the brain. 



Almost all the views entertained regarding the sympathetic vibrations 

 of the membrana tympani have been applied to the membrane of the 



1 Hist, et Mem. de la Societe Royale de Medecine, torn. iii. 



