THE OSSEOUS LABYRINTH. 



101 



external semicircular canal (fig. 108, 4 ; fig. 109. 7) arches horizontally outwards, 

 and opens by two distinct orifices into the upper and back part of the vestibule 

 (fig. 107, amp.e. and s.c.e.). The canal is shorter than either of the other two 

 (15 mm.) : its ampulla is at the oucer end, above the fenestra ovalis. 



Fig. 108. RIGHT BONY LABYRINTH, VIEWED FROM THE OUTER SIDE (after Soramerring). 



Magnified 2 times. 



The specimen here represented is prepared by separating piecemeal the looser substance of the 



petrous bone from the dense walls which immediately enclose the labyrinth. 1, the vestibule : 



2, fenestra ovalis ; 3, superior 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. 109. 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 opening of the superior 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. 



The cochlea (figs. 108 to 116), when cleared of the surrounding less dense bony 

 substance in which it lies embedded, appears in the form of a blunt cone, the base 

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



Fig. 110. VIEWS OF A. CAST OF THE INTERIOR OF THE LABYRINTH. (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, 

 nnd e, the external semicircular canals ; a, their several ampullae ; r e, fovea hemi-elliptica of the 

 vestibule ; r s, fovea hemispherica ; a v, aqueduct of the vestibule ; f o, fenestra ovalis ; f r, fenestra 

 rotunda ; c, the coiled tube of the cochlea ; c', the tractus spiralis foraminulentus. 



outwards, with an inclination forwards and downwards, and is close to the canal for 

 the tensor tympani muscle. It measures about 5mm. from base to apex, and mm. 

 in breadth at the base. It consists of a gradually tapering spiral tube, the inner 

 \vall of which is formed by a central column, or modiohis (fig. 113), around which 



