TIIK [NTEENAL KAR 



701 



TlIK Conil. I \ 



The cochlea, like the vest ihiilar portion of the internal ear, consists 

 of a bony case which incloses a membranous organ. 



Structure. The bony cochlea possesses a peculiar Hat pyramidal 

 shape. The base of the pyramid is in contact with the anterior aspect 

 of the vestibule; its apex or cii/iula. is directed forward, outward, and 

 slightly downward. The pyramid is hollow and contains in its axis a 

 conical bony support, the iinx/lotnx, which tapers from a broad base to 

 a pointed apex beneath the broader, 

 blunt, and rounded cupola of the 

 outer bony wall. The modiolus con- 

 tains a broad canal which receives 

 the cochlear division of the acoustic 

 nerve as it enters from the internal 

 meatus. 



The outer surface of the modio- 

 lus supports a bony shelf, the lam- 

 ina spiralis ossea, which winds in a 

 spiral manner from its base to its 

 apex, and ends in a hook-like proc- 

 ess, the limit ul tix. This shelf only 



FIG. 585. AXIAL SECTION THROUGH 

 THE COCHLEA OF A FETAL CALF. 



a, internal acoustic meatus in which 

 is the cut end of the cochlear nerve as it 

 enters the modiolus. X 6. <After Kol- 

 liker.) 



partially spans ihe interval between 

 the modiolus and the outer wall of 

 the cochlea. In life the remaining 

 interval is completed by a firm fi- 

 brous membrane, the l><t*\htr membrane (lamina spiralis membranacea). 

 Thus the cylindrical canal of the cochlea, which is wound spirally around 

 the modiolus making two and one-half turns from the base to the apex, is 

 subdivided into two parallel longitudinal divisions, which are respectively 

 known as the scala veslilnili and the scala tympaui. They are so dis- 

 posed that in a given turn of the canal the former is always nearer the 

 apex, the latter nearer the base of the cochlea. According to Wieder- 

 sheim (18!>r>) the human cochlea lias nearly three turns, the pig four, 

 the cat three, the rabbit two and one-half, the ox three and one-half, 

 and cetacea one and one-half turns. 



The osseous lamina spiralis presents a grooved margin or su/fiix, 

 from the basal or tympanic lip of which the lamina basilaris is con- 

 tinued to the opposing surface of the bony wall. The lamina spiralis 

 ossea is hollowed out in a diploic manner for the transmission of the 



