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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



labyrinth (Figs. 275, 276) consists of a median part, the vestibule, which is pro- 

 longed posteriorly in the system of semicircular canals and anteriorly in the 

 cochlea. The vestibule is a space which measures about one-fifth of an inch 

 in diameter, and it is perforated in its outer wall by an oval opening known 

 as the fenestra ovalis. The semicircular canals are three tubes of circular 



FIG. 275. Right bony labyrinth, viewed from 

 outer side : the figure represents the appearance 

 produced by removing the petrous bone down to 

 the denser layer immediately surrounding the 

 labyrinth (from Quain, after Sommering) : 1, 2, 3, 

 the superior, posterior, and horizontal semicir- 

 cular canals ; 4, 5, 6, the ampullae of the same ; 

 7, the vestibule ; 8, the fenestra ovalis ; 9, fenestra 

 rotunda ; 10, first turn of the cochlea ; 11, second 

 turn ; 12, apex. 



FIG. 276. Interior view of left bony labyrinth after 

 removal of the superior and external walls (from 

 Quain, after Sommering) : 1, 2, 3, the superior, pos- 

 terior, and horizontal semicircular canals ; 4, fovea 

 hemi-elliptica ; 5, fovea hemispherica ; 6, common 

 opening of the superior and posterior semicircular 

 canals ; 7, opening of the aqueduct of the vestibule ; 

 8, opening of the aqueduct of the cochlea; 9, the 

 scala vestibuli ; 10, scala tympani ; the lamina spiralis 

 separating 9 and 10. 



section, known respectively as the anterior or superior, the posterior, and the 

 external or horizontal semicircular canal. Their planes are at right angles to 

 one another, so that they occupy the three possible dimensions of space. The 

 external canal lies in a nearly horizontal plane, while the other two approach 

 the vertical. Each canal is dilated at one extremity into a globular cavity 



which is more than twice the diameter of the 

 canal itself, and which is known as the am- 

 pulla. The anterior and posterior canals 

 unite near the ends not provided with am- 

 pullae, and they enter the vestibule as a com- 

 mon tube. Anteriorly the cavity of the 

 -4 vestibule is continued as a tube of complex 

 internal structure which is coiled upon itself 



FIG. 277. Diagram of the osseous cochlea m * 



laid open (after Quain) : i, scala vestibuli ; two and one-half times, and which, from its 



2, lamina spiralis ; 3 scala tympani ; 4, cen- regemb ] ance to the sne ]l o f a snai l is known 

 tral pillar or modiolus. ' 



as the cochlea (PI. 2, Fig. 3). The osseous 



cochlea may be conceived as formed by a bony tube turned about a bony central 

 pillar, the modiolus, which diminishes in diameter from the base to the apex 

 of the cochlea. From the modiolus a bony shelf stretches into the cavity of 

 the tube, incompletely dividing it into two tubular chambers, winding round 

 the modiolus like a circular staircase, the upper of which chambers we shall 



