THE SENSE OF HEARING 



the semicircular canals and the cochlea. It is separated from the cavity of 

 the middle ear by an osseous partition which presents near its center an oval 

 opening, the foramen ovale. (See Fig. 319.) In the living condition this 

 opening is closed by the base of the stapes bone, which is held in position by 

 an annular ligament. The inner wall presents a 

 number of openings for the passage of nerve- 

 fibers. 



The Semicircular Canals. The semicircular 

 canals are three in number, and named from their 

 position, the superior vertical, the posterior verti- 

 cal and the horizontal. These canals are at 

 right angles one to the other and open by five 

 orifices into the vestibule, one of the orifices, how- 

 ever, being common to two of the canals. Each 

 canal near the vestibular orifice is enlarged to 

 almost twice the size of the rest of the canal form- 

 ing what has been termed the ampulla. The 

 superior canal is placed transversely to the long 

 axis of petrous portion of the temporal bone; the 

 posterior canal is placed almost parallel to the posterior surface of the petrous 

 portion of the temporal bone. The lateral or horizontal canal is placed 

 horizontally to the vertical canals. Their relation to one another on opposite 

 sides of the median plane of the body is shown in Fig. 318. From the ana- 

 tomic relations of these canals, it is apparent that the horizontal or lateral 

 canals lie in one and the same plane; that the two superior vertical canals, 

 as well as the two posterior vertical canals, lie in planes which are at right 

 angles to each other, and that the superior vertical canal of the one side, 



FIG. 317. M, THE TENSOR 

 TYMPANI MUSCLE THE Eus- 

 TACHIAN TUBE (LEFT). 

 (Landois). 



FIG. 318. DIAGRAMS OF SEMICIRCULAR CANALS TO SHOW THEIR^POSITION IN THREE PLANES 

 AT RIGHT ANGLES TO ONE ANOTHER. It will be seen that the two horizontal canals lie in the 

 same plane (H), and that the superior vertical of one side (SV) is in the same plane as the 

 posterior vertical (PV) of the other side. (from Ewald.) 



lies in a plane parallel to the plane in which the posterior vertical canal of 

 the opposite side lies. 



The Cochlea. The cochlea, the anterior portion of the labyrinth, is a 

 gradually tapering canal, about 35 mm. in length, wound spirally two and a 

 half times around a central bony axis, the modiolus. The cavity of the coch- 

 lea is partially subdivided into two cavities by a thin spiral plate of bone 

 which projects from the inner wall, known as the lamina ossea spiralis. In 

 the natural condition this partition is completed by a connective-tissue mem- 

 brane, so that the two passages are completely separated from each other. 



