OSSEOUS LABYRINTH. 



843 



AUEIS INTEKNA. 



The internal ear or essential part of the organ of hearing is situated in the 

 substance of the petrous part of the temporal bone, and consists of two sets of 

 structures, viz. : (1) a series of passages hollowed out of the bone and constituting 

 the osseous labyrinth; these are continuous with each other, and are named 





Superior semicircular canal 

 Ampulla of superior 

 semicircular canal 



Canalis facialis 



Recessus ellipticus 



Crista vestibuli 

 Recessus sphsericus 



Cochlea 



Fenestra cochleae 



Fenestra vestibuli 

 Ampulla of posterior semi- 



circular canal 



Ampulla of lateral semi- I 

 circular canal | 

 Lateral semicircular canal 



Posterior semi- 

 circular canal 



Crus commune 



Scala tympani 

 Lamina spiralis ossea 



Scala vestibuli 

 Opening of aquseductus 

 cochleae 



Fenestra cochleae 

 Recessus cochlearis 



Posterior 



circular 

 canal 



| Opening of crus commune 

 Opening of aquseductus vestibuli 



FIG. 716. LEFT BONY LABYRINTH 

 (viewed from the lateral aspect). 



FIG. 717. INTERIOR OP LEFT BONY LABYRINTH 

 (viewed from lateral aspect). 



from before backwards the cochlea, vestibule, and semicircular canals (Figs. 716, 

 717) ; (2) a complex arrangement of membranous channels (Fig. 720), situated 

 within, but not nearly filling, the bony labyrinth and forming the membranous 

 labyrinth. These channels are named the ductus cochlearis, the utricle, the saccule, 

 and the semicircular ducts ; the utricle and saccule are lodged within the vestibule. 



LABYKINTHUS OSSEUS. 



Vestibulum. The vestibule is the central portion of the osseous labyrinth, and 

 communicates behind with the semicircular canals and in front with the cochlea. It 

 is somewhat ovoid in shape, its long axis being directed forwards and lateralwards ; 

 it measures about 6 mm. antero-posteriorly, 4-5 mm. vertically, and about 3 mm. 

 transversely. Its lateral wall is directed towards the tympanic cavity, and in it 

 is the fenestra vestibuli, which is closed by the base of the stapes. Its medial 

 wall corresponds with the bottom of the internal acoustic meatus, and presents, at 

 its antero-inferior part, a rounded depression, the recessus sphaericus, which lodges 

 the saccule. This recess is perforated by twelve or fifteen small foramina (macula 

 cribrosa media), which transmit the filaments of the acoustic nerve to the saccule. 

 The recessus sphsericus is limited above and behind by an oblique ridge, the crista 

 vestibuli, the anterior extremity of which is triangular in shape and named the 

 pyramis vestibuli. Posteriorly this crista divides into two limbs, between which is 

 a small depression, the recessus cochlearis, perforated by about eight small fora- 

 mina, which give passage to the nervous filaments supplying the vestibular end of 

 the ductus cochlearis. Above and behind the crista vestibuli, in the roof and 

 medial wall of the vestibule, is an oval depression, the recessus ellipticus, which 

 lodges the utricle. The pyramis vestibuli and adjacent part of the recessus ellipticus 

 are perforated by twenty-five or thirty small apertures (macula cribrosa superior). 

 The foramina in the pyramis vestibuli transmit the nerves to the utricle ; those in 

 the recessus ellipticus, the nerves to the ampullse of the superior and lateral 

 semicircular ducts. Behind and below the recessus ellipticus is a furrow, gradually 

 deepening to form a canal, the aquseductus vestibuli, which passes backwards through 

 the petrous part of the temporal bone, and opens, as a slit-like fissure, about 

 midway between the internal acoustic meatus and the groove for the transverse 

 sinus. This aqueduct measures 8-10 mm. in length, and gives passage to the 



