C42 



THE EAR. 



behind the vestibule, into Trhich they open by five apertures, the con- 

 tiguous ends of 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 ; 



Fi" 452. 



Fig. 453. 



Fig. 452. — Right Bony Labyrinth, vibwed from the Outer Side (after 



Sommerring). ^ 



The specimen here represented is prepared by separating piecemeal the looser sub- 

 stance of the petrous bone from the dense walls which immediately enclose the laby- 

 rinth. 1, the vestibule ; 2, fenestra ovalis ; 3, sujierior semicircular canal ; 4, horizontal 

 or external canal ; 5, posterior canal ; * * *, ampullae of the semicircular canals ; 

 6, first turn of the cochlea ; 7, second turn ; 8, apex ; 9, fenestra rotunda. The 

 smaller figure in outline below shows the natural size. 



Fig. 453. — ^ViEW OP the Interior op the Left Labyrinth (from Sommerring). -^ 



The bony wall of the labyrinth is removed superiorly and externally. 1, fovea 

 hemi-elliptica ; 2, fovea hemispherica ; 3, common ojiening of the sujjerior and posterior 

 semicircular 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 

 tympani) ; 9, opening of the aqueduct of the cochlea ; 10, placed on the lamina spiralis 

 in the scala vestibuli. 



and each presents, at one end, a slightly dilated part, called the am^mlla. 

 The canals are compressed laterally, and measure about J„th of an inch 

 across ; but in the ampulla each has a diameter of y^gth of an inch. 



The canals differ from one another in direction, in length, and in 

 position with regard to the vestibule. The str-perm' semicircular canal 

 (fig. 452, 3, fig. 453, 5) is vertical and transverse, and it rises higher than 

 any other part of the labyrinth ; its place is indicated by a smooth 

 arched projection on the upper surface of the petrous bone. The 

 ampullary end of this canal is the external and anterior, and opens 

 by a distinct orifice into the upper part of the vestibule ; whilst the 

 opposite extremity joins the non-dilated end of the posterior semicir- 

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

 ■part of the vestibule (fig. 453, 3). The posterior semicircular canal 

 (fig. 452, 5, fig. 453, 6), 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 terminates in the 

 common canal above described. The external semicircular canal (fig. 

 452, 4, fig. 453, 7) arches horizontally outwards, and opens by two 

 diBtinct orifices into the upper and back part of the vestibule. The 



