THE SENSE OF HEARING- 



193 



and the free fenestra rotunda (cochleae), closed by a membrane 

 known as the secondary membrane of the tympanum, which is 

 directly connected with the scala tympani of the cochlea. 



The internal ear or labyrinth contains in two distinct parts 

 the sensory end-organs innervated by the two branches of the 



C. 



FIG. 73. Cast of the interior of the labyrinth. Left human ear. \. (From Henle.) A, seen from 

 outer side ; B, from inner side ; C, from above ; s., superior ; p., posterior ; e, external or 

 lateral semicircular canals ; a., their ampullae ; a.v., aqueduct of the vestibule ; f.o., fenestra 

 ovalis (vestibuli) ; f.r., fenestra rotunda (cochleae) ; c., spiral tube of cochlea. 



eighth nerve : the utricle, saccule, and the three semicircular 

 canals with their respective ampullae innervated by the vestibular 

 nerve, and the cochlea, innervated by the cochlear nerve. The 

 osseous labyrinth (Fig. 73), hollowed out of the petrous bone, must 



FK;. 74. Left membranous labyrinth, viewed externally. (Merkel.) Co., cochlea; DC., ductus 

 cochlearis ; Sac., saccule ; Utr., utricle ; s., e., p., superior, external, and posterior semicircular 

 canals ; a.v,, aquaeductus vestibuli ; C.r., canalis reuniens. 



be distinguished from the membranous labyrinth (Fig. 74), which 

 lies within it. The space between the two is filled with perilymph, 

 while the interior of the membranous labyrinth contains the 

 endolymph. 



The cochlea is the acoustic portion of the labyrinth ; it consists 

 of a spiral tube, divided into two chambers by a bony septum 

 (lamina spiralis) completed by a membranous portion (membrana 

 spiralis). The lower chamber or scala tympani communicates by 

 the fenestra rotunda with the tympanum ; the upper chamber, 



VOL. IV 



