HEARING. THE MIDDLE EAR. 193 



However this may be, the principal use of the air in the 

 cavity of the tympanum is not to transmit the vibrations of 

 that membrane; but to balance the pressure of the atmosphere 

 on its external surface, and thus to render it completely in- 

 dependent between two equal pressures. This is effected by 

 means of the Eustachian tube, which conducts the air into 

 the middle ear. The temporary obstruction of this canal 

 induces buzzings in the ear, causing temporary deafness, 

 which is intensified by its entire obliteration. The canal 

 also serves as an outlet for mucus and other fluids, which 

 may be secreted in the cavity of the tympanum. 



The sonorous waves enter the vestibule by the fenestra 

 ovalis; this opening is closed by the base of the stapes, and 

 receives the vibrations from the chain of little bones. The 

 membrane of the fenestra rotunda transmits to the scala 

 tympani of the cochlea the aerial vibrations of the cavity of 

 the tympanum. This membrane, as Scarpa has remarked, is 

 a secondary tympanum. 



On reaching the labyrinth the vibrations are propagated to 

 the fluid which bathes it, and thus reach the membranous 

 labyrinth, and the scala vestibuli of the cochlea, where finally 

 they encounter the extremities of the ramifications of the 

 auditory nerve. 



Besides the sonorous aerial waves, the ear perceives, as 

 has already been stated, those which have been caused by 

 an impression on the bones of the skull. Thus, when a 

 sonorous body is held between the teeth, or against the walls 

 of the cranium, the sound is perceived by the auditory 

 apparatus. It is in this way that, in spite of the loss of the 

 tympanum and the small bones of the ear, some persons can 

 still perceive sounds of external origin. But it is indispens- 

 able that the membranes of the fenestra ovalis and fenestra 

 rotunda, which close the openings of the labyrinth into the 

 cavity of the tympanum, should still remain perfect, and that 

 the fluid of the labyrinth should still bathe its cavities. But, 

 as may be easily conceived, hearing under these circumstances 

 is very limited, since it can only take place when the sonor- 

 ous body is in contact with the bones of the head. 



The functions of the three divisions of the labyrinth have 



