THE OSSEOUS LABYRINTH. 



C43 



canal is shorter than either of the other two : its ampulla is at tlie 

 outer end, above the fenestra ovalis. 



Fifr. 451. 



Fi- 455. 



Fig. 454. — Views of a Cast of the Interior of the Labvrinth (from Henle). f 



Such casts may easily be made in fusible metal, and' give a very correct view of the 

 form of the different parts of the labyrinthic cavity. A, view of the left labyrinth from 

 the outer side ; B, the right labyrinth from the inner side ; C, the left labyrinth from 

 above ; s, the superior, p, the posterior, and e, the external semicircular canals ; «, their 

 several ampuUis ; r e, fovea hemi-elliptica of the vestibule ; ?• s, fovea hemispherica ; a t', 

 aqueduct of the vestibule ; / o, fenestra ovalis ; / r, fenestra rotunda ; c, the coiled 

 tube of the cochlea ; c, the first part of tube towards the base with the tractus forami- 

 nosus spiralis. 



The cochlea (fig. 452, 6) is the most anterior division of the internal 

 ear. When cleared of the surrounding less dense bony substance in which 

 it lies imbedded, it presents the form of a blunt cone, the base of which is 

 turned towards the internal auditory meatus, whilst the apex is directed 

 outwards, with an inclination forwards and downwards, and is close ta 

 the canal for the tensor tympani muscle. It 

 measures about a quarter of an inch in length, and 

 the same in breadth at the base. The osseous 

 cochlea consists of a gradually tapering spiral tube, 

 the inner wall of which is formed by a central 

 column, or modiolus (fig. 456, 1), around which it 

 winds. It is partially divided along its whole extent 

 by a spiral lamina(2), projecting into it from the mo- 

 diolus. From this osseous spiral lamina membran- 

 ous structures are in the recent condition stretched 

 across to the outer wall of the tube, and thus com- 

 pletely separate two passages or scake, one on each 

 side of the spiral lamina, which communicate one 

 Avith the other only by a small opening, named 

 Itelicotrema, placed at the apex of the cochlea. 



That the cochlea is justly to be considered as an 

 elongated tube, coiled spirally on the modiolus, is 

 illustrated by the simple pouch- like form of the 

 rudimentary cochlea of birds (fig. 455) as well as by 

 the history of its development. 



The spiral osseous canal is about an inch and 

 a half long, and about the tenth of an inch 

 in diameter in its widest part (at the com- 

 mencement). From this point the canal makes two turns and a half 

 around the central pillar (from left to right in the right ear, and in the 

 opposite direction in the left ear), and ends by an arched and closed 

 extremity called the cupola, which forms the summit of the cochlea. 



T T 2 



Fig. 455. — Osseous 

 Labyrinth op the 

 Barn Owl (Strix 

 Flammea) (from 

 Breschet). * 



1, semicircular 

 canals ; 2, vestibule ; 

 3, cochlea in the form 

 of a short straight 

 tube. 



