THE SENSE OF HEARING. 255 



caused by an unequal pressure of the air in the middle ear can be 

 shoWn 



1. By closing the mouth and nose and forcing air from the lungs 

 through the Eustachian tube into the ear, producing an increase 

 in pressure. 



2. By closing the nose and mouth, and making efforts at deglutition, 

 which withdraws the air from the ear and diminishes its pressure. 

 In both instances the free vibrations of the tympanic membrane 



are interfered with. The pharyngeal orifice of the Eustachian tube 

 is opened by the action of certain of the muscles of deglutition 

 viz., the levator palati, the tensor palati, and the palato-pharyngei 

 muscles. 



The internal ear, or labyrinth, is located in the petrous portion 

 of the temporal bone, and consists of an osseous and a membranous 

 portion. 



The osseous labyrinth is divisible into three parts viz., the vesti- 

 bule, the semicircular canals, and the cochlea. 



The vestibule is a small, triangular cavity, which communicates 

 with the middle ear by the foramen ovule ; in the natural condition 

 it is closed by the base of the stapes bone. The filaments of the 

 auditory nerve enter the vestibule through small foramina in the 

 inner wall, at the fovea hemispherica. 



The semicircular canals are three in number, the superior vertical, 

 the inferior vertical, and the horizontal, each of which opens into 

 the cavity of the vestibule by two openings, with the exception of 

 the two vertical, which at one extremity open by a common orifice. 



The cochlea forms the anterior part of the internal ear. It is a 

 gradually tapering canal, about i T / 2 inches in length, which winds 

 spirally around a central axis, the modiolus, two and one half times. 

 The interior of the cochlea is partly divided into two passages by 

 a thin plate of bone, the lamina osseous spiralis, which projects 

 from the central axis two thirds of the way across the canal. These 

 passages are termed the scala vestibuli and the scala tympani, from 

 their communication with the vestibule and tympanum The scala 

 tympani communicates with the middle ear through the foramen ro- 

 tundum, which, in the natural condition, is closed by the second 

 membrana tympani ; superiorly they are united by an opening, the 

 helicotrema. 



The whole interior of the labyrinth, the vestibule, the semi- 



