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TEXT-BOOK OF PHYSIOLOGY. 



cular 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. 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 (Fig. 340). 



The semicircular canals are three in number, 

 each at right angles to the other two, a superior 

 vertical, an inferior vertical, and a horizontal, 

 each of which opens by two orifices into the 

 cavity of the vestibule, with the exception of the 

 two vertical, which unite at one extremity and 

 then open by a single orifice. Each canal near 

 its vestibular orifice in enlarged to almost twice 

 the size of the rest of the canal, forming what 

 is known as the ampulla. 



The cochlea, the anterior portion of the laby- 

 rinth, 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 cochlea 

 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. 

 The upper passage or scala is in free communication with the vestibule, 

 and is known as the scala vestibuli; the lower passage or scala in the dead con- 



FIG. 339. M, THE TENSOR 

 TYMPANI MUSCLE THE Eus- 

 TACHIAN TUBE (LEFT). 

 (Landois.) 



FIG. 340. BONY COCHLEA, i. 

 Ampulla of superior semicircular 

 canal. 2. Horizontal canal. 3. 

 Junction of superior and posterior 

 semicircular canals. 4. The pos- 

 terior semicircular canal. 5. Fora- 

 men rotundum. 6. Foramen ovale. 

 7. Cochlea. 



FIG. 341. i. Utricle. 2. 

 Saccule. 3. Vestibular end of 

 cochlea. 4. Canalis reuniens. 

 5. Membranous cochlea. 6. 

 Membranous semicircular 



canals. 



dition communicates with the tympanum by means of a round opening, the 

 foramen rotundum, and is therefore known as the scala tympani. In the liv- 

 ing condition this opening is completely closed by a membrane, a second 

 membrana tympani. Both the scalae vestibuli and tympani communicate 

 at the apex of the cochlea by a small opening, the helicotrema. The 

 modiolus, the central bony axis, is perforated from base to apex by 

 a canal for the passage of the auditory nerve-fibers; lateral canals, diverg- 



