THE OSSEOUS COCHLEA. 817 



For the like dissection in the recent state, a softened bone should ^ d recent 

 be used. 



The cochlea (fig. 303) is conical in form, and is placed almost form and 

 horizontally in front of the vestibular space. The base of this body si 

 is turned to the meatus auditorius internus, and is perforated by 

 small apertures ; while the apex is directed to the inner wall of the 

 tympanum, opposite the canal for the tensor muscle. Its length is size ; 

 about a quarter of an inch, and its width at the base is about the 

 same. Resembling a spiral shell, the cochlea consists of a tube resembles a 

 wound round a central part or axis ; but it differs from the shell in j^ome 6 

 having its tube subdivided by a partition. respects. 



The axis or modiolus (fig. 303, a) is the central stem which Central 

 supports the windings of the spiral tube. Conical in shape, its size axis* 1 " C 



FIG. 303. SECTION THROUGH THE COCHLEA (BRESCHET). 



a. Branches of the auditory nerve, /. Hiatus or helicotrema. 

 contained in the canals of the axis. s.t. Scala tympani. 



b. Enlarged upper end of the axis. s.v. Scala vestibuli. 



c. Septum of the cochlea. s.m. Scala media or canal of the 

 e. Membrane of Reissner. cochlea. 



diminishes rapidly towards the last half turn of the tube, but it j g conical, 

 enlarges at the tip of the cochlea, forming a second small cone (6), 

 which is bent. The axis is perforated by canals as far as the con- and porous, 

 tracted part of the last half-turn, and the central one is larger than 

 the others ; these transmit vessels and nerves in the fresh state. 



The spiral tube forms two turns and a half round the stem, and A spiral 

 terminates above in a closed extremity named the cupola. When J" o 

 measured along the outer side, it is about one and a half inch long, forms 2 



Its diameter at the beginning is about one-tenth of an inch, but it 

 ,.... i i *_ * i 11 -i measure 



diminishes gradually to halt that size towards the opposite end. ment; 



Of the coils that the tube makes, the first is much the largest ; coils, 

 this projects towards the tympanum, and gives rise to the eminence 

 of the promontory on the inner wall of that cavity. The second 

 turn is included within the first coil. The last half-turn bends 

 sharply round, and presents a free margin (b) the edge of the axis. 



D.A. 3 G 



