THE OSSEOUS COCHLEA, 



755 



circular canal arches horizontally outwards, and opens by two distinct 

 orifices into the upper and back part of the vestibule. This canal is shorter 

 than either of the other two : its ampulla is at the outer end, just above the 

 fenestra ovalis. 



Fig. 509. 



Fig. 509. VIEWS OF A CAST OP THE INTERIOR OP THE LABYRINTH (from Henle). f 



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

 form of the different parts of the labyrmthic 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 ; a, their 

 several ampullae ; re, fovea hemi-elliptica of the vestibule ; rs, fovea hemispherica ; av, 

 aqueduct of the vestibule ; fo, fenestra ovalis ; fr, fenestra rotunda ; c, the coiled tube 

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

 nosus spiralis. 



The cochlea is the most anterior division of the internal ear. When the 

 dense bony substance, in which it lies embedded, is picked away, the 

 cochlea 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 to 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 part of the cochlea con- 

 sists of a gradually tapering spiral tube, the inner wall of which is formed 

 by the central column, or modiolus, round which it 

 winds, and which is partially divided along its whole 

 extent by a spiral lamina, projecting into it from the 

 modiolus. From this osseous spiral lamina membranous 

 structures are stretched across to the outer wall of the 

 tube, and thus are completely separated two passages 

 or scalee, oce on each side of the spiral lamina, which 

 communicate one with the other by only a small open- 

 ing, named helicotrema, placed at the apex of the 

 cochlea. 



Fig. 510. OSSEOUS LABYRINTH OF THE BARN-OWL (SiRix 

 FLAMMEA) (from Breschet). * 



Fig. 510. 



1, semicircular canals ; 2, vestibule ; 

 of a short straight tube. 



cochlea in the form 



That the cochlea is justly to be considered as an elongated tube, coiled 

 srirally on the modiolus, is illustrated by the simple pouch-like form of the 

 rudimentary cochlea of birds. 



The spiral canal of the cochlea is about an inch and a half long, and 

 about the tenth of an inch in diameter in its widest part at the commence- 



3 D 2 



