751 



THE EAR. 



inwards : except in the last-mentioned direction, in which it is somewhat 

 smaller, it measures about -^th of an inch in diameter. 



The outer wall which separates it from the cavity of the tympanum, is 

 perforated by the fenestra ovalis, which in the recent state is closed by the 

 base of the stapes and its annular ligament. 



At the fore part of the inner wall is a small round pit, the fovea hemi- 

 spherica, pierced with many small holes, which serve to transmit branches of 

 the auditory nerve from the internal auditory meatus. This fossa is limited 

 behind by a vertical ridge named crista vestibuli or eminentia pyramidalis. 

 Behind the crest is the small oblique opening of a canal, the aqueduct of the 

 vestibule, which extends to the posterior surface of the bone, and transmits 

 a small vein in a tubular prolongation of membrane. 



In the roof is an oval depression, placed somewhat transversely, fovea 

 hemi-elliptica, whose inner part is separated by the crest from the hemi- 

 spherical fossa. 



At the back part of the vestibule are five round apertures, leading into 

 the semicircular canals : and at the lower and fore part of the cavity is a 

 larger opening, which communicates with the scala vestibuli of the cochlea 

 apertura scalm vestibuli. 



The semicircular canals are three bony tubes, situate above and behind 

 the vestibule, into which they open by five apertures, the contiguous ends of 



Fig. 508. 



Fig. 508. VIEW OF THE INTERIOR OF THE 

 LEFT LABYRINTH (from Sommerring). ?i 



The bony wall of the labyriuth is removed 

 superiorly and externally. 1, fovea hemi- 

 elliptica ; 2, fovea hemispherica ; 3, common 

 opening of the superior and posterior semi- 

 circular canals ; 4, opening of the aqueduct 

 of the vestibule ; 5, the superior, 6, the 

 posterior, and 7, the external semicircular 

 canals; 8, spiral tube of the cochlea (scala 

 tyropani) ; 9, opening of the aqueduct of the 

 cochlea ; 10, placed on the lamina spiralis 

 in the scala vestibuli. 



two of the canals being joined. They 

 are unequal in length, but each tube 



is bent so as to form about two-thirds of a circle ; and each presents, at one 

 end, a slightly dilated part, called the ampulla. The canals are compressed 

 laterally, and measure across about ^th of an inch ; but in the ampulla each 

 has a diameter of -j^-th of an inch. 



The canals differ from one another in position with regard to the vestibule, 

 in direction, and in length. The superior semicircular canal is vertical and 

 transverse ; and, rising above any other part of the labyrinth, its place is 

 indicated by a smooth arched projection on the upper surface of the 

 bone. The ampullary end of this canal is the anterior, and opens by 

 a distinct orifice iuto the upper part of the vestibule ; whilst the oppo- 

 site extremity joins the non-dilated end of the posterior semicircular 

 canal, and opens by a common aperture with it into the back part of 

 the vestibule. The posterior semicircular canal, vertical and longitudinal 

 in direction, is the longest of the three tubes: its ampullary end is 

 placed at the lower and back part of the vestibule ; and the opposite 

 end joins in the common canal above described. The external semi- 



