THE SENSE OF HEARING. 171 



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, 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 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 ovale; 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 hemi- 

 spherica. 



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 one and a half inches in length, which winds spirally 

 around a central axis, the modiolus, two and a 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 

 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 fat foramen 

 rotundum, which, in the natural condition, is closed by the second mem- 

 brana tympani ; superiorly they are united by an opening, the helicotrema. 



The whole anterior of the labyrinth, the vestibule, the semicircular 

 canals, and the scala of the cochlea, contains a clear, limpid fluid, the peri- 

 lymph, secreted by the periosteum lining the osseous walls. 



The Membranous Labyrinth corresponds to the osseous labyrinth 

 with respect to form, though somewhat smaller in size. 



The Vestibular portion consists of two small sacs, the titricle and saccule. 



The Semicircular canals communicate with the utricle in the same 

 manner as the bony canals communicate with the vestibule. The saccule 

 communicates with the membranous cochlea by the canalis reuniens. In 

 the interior of the utricle and saccule, at the entrance of the auditory nerve, 



