THE INTERNAL EAR 



701 



THE COCHLEA 



The cochlea, like the vestibular portion of the internal ear, consists 

 of a bony case which incloses a membranous organ. 



Structure. The bony cochlea possesses a peculiar flat pyramidal 

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

 of the vestibule; its apex or cupola is directed forward, outward, and 

 slightly downward. The pyramid is hollow and contains in i'ts axis a 

 conical bony support, the modiolus, 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 

 meat us. 



The outer surface of the modio- 

 lus supports a bony shelf, the lam- 

 inn vjnralis ossea, which winds in a 

 spiral manner from its base to its 

 apex, and ends in a hook-like proc- 

 ess, the Tiamulus. This shelf only 

 partially spans the 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 basilar 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 vestibuli and the scala tympani. 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 (1893) the human cochlea has 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 sulcus, 

 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 



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.) 



