1140 



THE ORGANS OF SPECIAL SENSE 



petrous bone; it is the longest of the three, measuring from 18 to 22 mm. ; its ampul- 

 lated end commences at the lower and back part of the vestibule, its opposite end 

 joining to form the common canal already mentioned. In the wall of the ampulla 

 of the posterior canal are a number of small openings (macula cribrosa inferior) 

 for the passage of nerves to the ampulla. 



The lateral or horizontal canal Qtanalis semicircularis lateralis) is the shortest 

 of the three. It measures from 1 2^o 15 mm., and its arch is directed outward 

 and backward; thus each semicircular canal stands at right angles to the other 

 two. Its ampullated end corresponds to the upper and outer angle of the vesti- 

 bule, just above the fenestra ovalis, where it opens close to the ampullary end of 

 the superior canal; its opposite end opens by a distinct orifice at the upper and 

 back part of the vestibule. 



The cochlea (Figs. 846 and 847) bears some resemblance to a common snail- 

 shell ; iLiojTOS_Jhe_anJ^ the labyrinth, is conical in form, and placed 

 almost horizontally in front of the vestibule; its agex (cupula) is directed forward 

 and outward, with a slight inclination downward, toward the upper and front part 



FIG. 847. Osseous cochlea in vertical section. The broken, white lines indicate the position of the basilar 

 membrane of the canal of the cochlea. Semidiagrammatic. (Testut.) 



of the inner wall of the tympanum; its base (basis cochleae) corresponds with the 

 anterior depression at the bottom of the internal auditory meatus, and is perforated 

 by numerous apertures for the passage of the cochlear divisions of the auditory 

 nerve. It measures nearly a quarter of an inch (5 mm.) from base to apex, and 

 its breadth across the base is somewhat greater (about 9 mm.). It consists of 

 a conical-shaped central axis, the modiolus; of a canal, the bony canal of the cochlea, 

 the inner wall of which is formed by the central axis, wound spirally around it for 

 two turns and three-quarters, from the base to the apex, and of a delicate lamina, 

 the lamina spiralis ossea, which projects from the modiolus, and, following the 

 windings of the canal, partially subdivides it into two. In the recent state a mem- 

 brane, the membrana basilaris. stretches from the free border of this lamina, to the 

 outer wall of the cochlea, and completely separates this canal into two passages, 

 which7 however, communicate with each other at the apex of the modiolus by a 

 small opening, named the helicotrema. 



The Modiolus (Figs. 848 and 849) is the central axis or pillar of the cochlea. 

 It is conical in form, and extends from the base to the apex of the cochlea. Its 

 base (basis modioli) is broad, and appears at the bottom of the internal auditory 



