INTERNAL EAR 



283 



apex they are united by the helicotrema, a small hole about 0.4 sq mm in 

 area. From this apex the two canals diverge with an approximate con- 

 stant increase in diameter, as shown in Fig. VII-9. The scala tympani 

 ends abruptly in a pocket just below the oval window. A round hole, 

 covered by a fibrous membrane, the round window, separates this termi- 

 nus of the scala tympani and the cavity of the middle ear. 



Superior 



Scala vestibuli 



/ 

 Pull of stapedius 



tes'or "*"„ ' 

 / 



Posterior 



vReissner's membrane 



' External spiral 

 ligament 



Anterior 



Aural 

 membrane 



Inferior 



Fig. VI 1-16. Diagrammatic section of the cochlea showing the relative positions 

 of the footplate of the stapes and the round window with respect to the scala media 

 and aural membrane. Note relative width of ligaments of superior-anterior edge of 

 footplate and relative size of footplate and round window with respect to position 

 of the scala media. 



The scala vestibuli is connected at its base to the saccule and utricle. 

 In the outer wall of the bony vestibule is inserted the fenestra vestibuli 

 (oval window) closed by the footplate of the stapes, whose annular mem- 

 brane permits motion of the footplate without allowing the escape of the 

 perilymph filling these canals (Fig. VII-16). 



The scala media is an endolymphatic tubular sac. At the apex of the 

 cochlea it ends in a blind extremity: its basal end forms a blind pouch 

 between the oval window and the round window. It is connected with 

 saccule and utricle by a small canal through which its hydrostatic pres- 

 sure is adjustable. The density of its fluid is 1.034 grams/cc at 37° C 

 and its viscosity 0.0197 cgs units (Bekesy [1933]). 



The floor of the scala media is the acoustic membrane. It is composed 



