THE EAR AND THE SENSE OF HEARING 



129 



projects from the central pillar a thin shelf of bone partly 

 dividing the tube into two parts. 



173. The Membranous Cochlea. From the edge of the 

 bony shelf of the cochlea two membranes reach to the 

 opposite wall of the cochlea and divide the cavity into 

 three spiral tubes (Fig. 77). One of these, the vestibular 



Fig. 77. Diagram of a cross section of a single coil of the cochlea. 



Showing the structures that compose the organ of Corti. 



passage (scala vestibuliy, communicates with the vesti- 

 bule. A second, the tympanic passage (scala tympani), 

 has its base against the membrane of the round window. 

 At the apex of the coil of the cochlea the vestibular and 

 tympanic passages communicate with each other (Fig. 74). 

 The middle passage, the membranous cochlea, or cochlcar 



MACY'S PHYS. 9 



