682 



DISSECTION OF THE EAR. 



G . 



O 



O 



the windings of the spiral tube. Conical in shape, its size diminishes 

 rapidly towards the last half turn of the tube, but it enlarges at the tip of 

 the cochlea, forming a second small cone (b), which is bent. The axis is 

 perforated by canals as far as the contracted part in the last half turn, and 

 the central one is larger than the others; these transmit vessels and 

 nerves in the fresh state. 



Fig. 246. 



S.7/Z, 



a. Axis with its canals. 

 6. Infundibulum or enlarged upper 

 end of the axis. 



c. Septum of the cochlea. 



d. Membrane of Corti. 



e. Membrane of Reissner. 

 /. Hiatus or helicotrema. 

 8. t. Scala tympani. 



s. v. Scala vestilmli. 



SBCTIOX THROUGH THE COCHLEA (Breschet). 



The spiral tube forms two turns and a half around the stem, and termi- 

 nates above in a closed extremity named the cupola. When measured 

 along the outer side, it is about one inch and a half long. Its diameter at 

 the beginning is about one-tenth of an inch, but it diminishes gradually to 

 half that size towards the opposite end. 



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

 jects towards the tympanum, and gives rise to the eminence of the pro- 

 montory 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 (6) (the edge of the axis). 



In the recent bone the tube is divided into two main passages (scalse) 

 by the septum (fig. 246, c). In the dry bone a remnant of this partition 

 is seen in the form of a thin plate bone, lamina spiralis, projecting from 

 the axis ; and on the outer wall is a slight groove, opposite the ridge of 

 bone. 



Septum of the spiral tube (fig. 247). The partition in the recent bone 

 dividing the tube of the cochlea into two passages, consists of an osseous 

 and a membranous portion : 



The osseous part ( s ), formed by the laminze spiralis, extends about half 

 way across the tube. Inferiorly it begins in the vestibule, where it is wide, 

 and is attached to the outer wall so as to shut out the fenestra rotunda from 

 that cavity ; and diminishing in size ends above in a point, the hamulus, 

 opposite the last half turn of the cochlea. Between the hamulus and the 

 axis a space, which is converted by the membranous part of the septum 

 into a foramen (fig. 246, f) (hiatus, helicotrema), and allows the inter- 

 communication of the two chief passages of the cochlear tube. 



The lamina spiralis is formed by two plates of the bone, which inclose 

 canals for vessels and nerves, and are separated farthest from each other at 



