THE LABYRINTH. 531 



The semicircular canals (Figs. 198, 199) are three arched 

 cylindriform bony canals, set in the substance of the petrous 

 bone. They all open at both ends into the vestibule (two of 

 them first coalescing). The ends of each are dilated just be- 



FlG. 199. 



Interior of the osseous labyrinth. V. Vestibule, av. Aqueduct of the vestibule, 

 o. Fovea hemielliptica. r. Fovea hemispherica. S. Semicircular canals. *. Su- 

 perior, p. Posterior, i. Inferior, a, a, a. The ampullar extremity of each. C. 

 Cochlea, ac. Aqueduct of the cochlea, sv. Osseous zone of the lamina spiralis, 

 above which is the scala vestibuli, communicating with the vestibule, st. Scala 

 tympani below the spiral lamina. From Scemmerring. 



fore opening into the vestibule ; and one end of each being 

 more dilated than the other is called an ampulla. Two of the 

 canals form nearly vertical arches ; of these the superior is also 

 anterior; the posterior is inferior; the third canal is horizontal, 

 and lower and shorter than the others. 



The cochlea (1, 2, 3, Fig. 198, and Fig. 200), a small organ, 

 shaped like a common snail shell, is seated in front of the ves- 

 tibule, its base resting on the bottom of the internal meatus, 

 where some apertures transmit to it the cochlear filaments of 

 the auditory nerve. In its axis, the cochlea is traversed by 

 a conical column, the modiolus, around which a spiral canal 

 winds with about two turns and a half from the base to the 

 apex. At the apex of the cochlea the canal is closed ; at the 

 base it presents three openings, of which one, already men- 



