282 AUDITORY BIOPHYSICS 



V. The tympanic tensor relaxes and the footplate shifts to its normal 

 position, producing a negative potential. 



Mechanically, the plate returns to its normal position in two steps. 

 The tympanic tensor relaxes, and the toe of the plate returns to its nor- 

 mal position. Then the restoring forces of the ligaments pull the heel 

 back to its normal position. The negative potential generated by this 

 double movement of the fluid in the cochlea should be electrically 

 separable, and it may account for the complexity of the fifth phase of 

 Wiggers' electrical response curve. 



The conclusion is that Wiggers' cochlear microphonic action data 

 parallel the possible motions of the stapes without violating any of the 

 mechanical degrees of freedom of its structural characteristics. 



Internal Ear 



The petrous portion of the temporal bone contains two mechanisms, 

 the vestibule and the cochlea. The vestibule consists of a series of 

 organs which concern the maintenance of positional equilibrium. The 

 anatomically closely related, though physiologically entirely separate, 

 bony cochlea is a two-and-one-half-turn hollow canal having a spirally 

 converging radius. This hollow spiral bony coil has a base whose diame- 

 ter is about 6 mm (Keen [1939]) ; its second turn is about 4 mm in diame- 

 ter, and the diagonal distance from its apex to the upper margin of the 

 oval window is about 6 mm; its altitude is about 5.5 mm. The spiral is 

 wound around a hollow cone of bone called the modiolus, along the axis of 

 which runs the auditory nerve. An inner spiral shelf of bone extends 

 about two thirds the way across the canal, dividing it nearly equally into 

 an upper and a lower portion (Fig. VII-16). Attached to the edge of 

 this bony shelf are two membranes connected to the outer wall of the 

 canal so that the upper canal is divided throughout its length into two 

 ducts. The duct nearest to the apex is called the scala vestibuli; the 

 triangular middle duct, limited by the two membranes and the outer 

 wall, is known as the scala media. The canal nearest to the base is called 

 the scala tympani. 



The gap between the inner spiral shelf and the outer wall is bridged by 

 the firm, fibrous basilar membrane. The very delicate non-vascular 

 membrane stretching from the spiral shelf obliquely outward to the bony 

 wall of the cochlea is called Reissner's membrane. The scala media has 

 for its diagonal ceiling the Reissner membrane and for its floor the 

 acoustic membrane supporting the cells of Hen sen and the organ of 

 Corti. 



The scala tympani below, and the scala vestibuli above, the scala 

 media extend spirally from the base to the apex of the cochlea. At the 



