608 THE NERVOUS SYSTEM. 



mm, long and 9 mm. broad at its base. It consists of an axis, 

 the modiolus or columella, which runs through the entire structure, 

 from the base to the apex ; around the modiolus runs the spiral 

 canal. At the base the raodiolus is perforated to transmit filaments 

 of the cochlear branch of the auditory nerve, and through it ex- 

 tends the canalis centralis modioli, which transmits a nerve and an 

 artery. 



The spiral canal is about 2 mm. in diameter and 3.3 cm. in 

 length. It makes two and three-fourth turns, clockwise i. e., 



FIG. 388. The cochlea and vestibule as seen from above: A, cochlea; B, vesti- 

 bule ; (7, internal auditory canal ; D, tympanum. 1, Lower border of fenestra ovalis ; 

 2, vestibulotympanic cleft; 3, fossa hemispherica ; 4, fossa hemi-elliptica ; 5, fossa 

 cochlearis ; 6, orifice of aqueduct of vestibule ; 7, lower orifice of posterior semi- 

 circular canal ; 8, non-amp ullary orifice of the external semicircular canal ; 9, scala 

 tympani ; 10, scala vestibuli ; 11, cupola ; 12, lamina spiralis, with 12', its vestibulary 

 origin ; 12", its external border ; 13, helicotrema (Testut). 



in the direction taken by the hands of a clock around the 

 modiolus. At the apex the canal terminates in the cupola. This 

 canal is partially divided into two by a bony septum, the lamina 

 spiralis, which consists of two thin plates of bone between which 

 are minute canals for the transmission of nerve-fibers. The 

 lamina spiralis extends from the modiolus only about half-way 

 toward the outer wall of the spiral canal. In the recent state 

 there is a membrane, the membrana basilaris, extending from the 

 edge of the lamina spiralis to the outer wall, dividing the canal 

 into two parts, the lower being the scala tympani, while the 



