THE EAR 



219 



pressure is generally equalised on the two sides of the drum. The Eustachian 

 tube being narrow, is easily blocked by the meeting of its opposite walls ; and 

 if this occurs at a moment when the pressures on the internal and external 

 surfaces of the drum are unequal, 'a peculiar sensation is experienced, and 

 hearing is for the time defective. But by performing the act of swallowing, 

 the tube is opened and the normal pressure restored. 



The internal ear consists of a very complex cavity hollowed 

 out of the bone (the osseous labyrinth}, which contains a similar 

 cavity (the membranous labyrinth} bounded by membranous walls. 



The osseous labyrinth consists of three parts : the vestibule 

 (Lat. vestibulum, an entrance), the semicircular canals, and the 

 cochlea (Lat. a snail). 



Fig. 209. The Osseous Cochlea 

 divided through the middle. 



Fig. 208. Interior of the Left 

 labyrinth. 



i, 2 and 3, semicircular canals ; 4, cochlea ; 

 5, vestibule. 



Fig. 210. Diagrammatic view of 

 the Osseous Cochlea laid open. 



The vestibule is the central chamber of the osseous labyrinth ; it commu- 

 nicates with the semicircular canals behind, and the cochlea in front. It is 

 separated in part from the tympanum by an opening the fenestra ovalis 

 (the oval window) to which we have already referred as being closed by the 

 broad part of the stapes. 



The semicircular canals are three tubes which communicate with the 

 vestibule by five openings. They are dilated at their ends, and two of them 

 unite at one extremity before entering the vestibule. 



The cochlea is a spiral tube, in shape like the shell of a snail. It consists of 

 two and a half turns, wound round a central pillar called the modiolus (Lat. the 

 nave of a wheel). A thin layer of bone projects from the modiolus, following 

 the convolutions of the cochlea. The cochlea communicates with the tym- 

 panum by an opening called the fenestra rotunda (round window), which is 

 closed by a membrane. 



The membranous labyrinth lies within the osseous labyrinth, and agrees 

 with it in general form. The space between them is filled with a fluid 



