THE SENSE OF HEARING. 



755 



todiolus. The two scalae communicate with one another only at 

 le top of the cochlea, by an opening: the hiatus, or helicotrema. 

 lat portion of the cochlear canal that is above the septum termi- 

 lates in the vestibule; hence scala vestibuli. The lower portion 

 )pens into the tympanum through the round window; hence scala 

 ipani. 



The membranous portion of the septum, or lamina spiralis, con- 

 sists of two layers: The superior layer is the membrane of Corti, or 

 irana tectoria; the other is the membrana basilaris. These two 

 nembranes are placed parallel with one another to contain between 

 them the organ of Corti, The latter rests upon the basilar mem- 

 )rane. 



Fig. 311. Scheme of the Labyrinth and Terminations of the Auditory 

 Nerve. ( LANDOIS. ) 



I. Transverse section of a turn of the cochlea. 



II. Ampulla of a semicircular canal, a, p, Auditory cells, p, Cell provided 

 with a fine hair. T, Otoliths. 



III. Scheme of the human labyrinth. 



IV. Scheme of a bird's labyrinth. 



V. Scheme of a fish's labyrinth. 



The bony portion of the septum has, upon its superior external 

 face, a denticulated, cartilaginous substance called the lamina den- 



ita. From the superior surface of the lamina spiralis, and 

 iternal to the lamina denticulata, exists a delicate membrane, the 

 ibrane of Reissner. This membrane divides the scala vestibuli 

 ito two passageways, one of which is the ductus cocJilearis. It con- 

 lins the essential portion of the auditory apparatus of the cochlea: 

 le organ of Corti. It forms part of the membranous labyrinth. 



The membranous labyrinth is a closed sac consisting of semi- 

 ocular canals, a vestibular portion, and the membranous part of 



