720 



SPECIAL SENSES 



General Arrangement of the Bony Labyrinth. The internal portion 

 of the auditory apparatus is contained in the petrous portion of the tem- 

 poral bone. It consists of an irregular cavity, called the vestibule, 

 the three semicircular canals (13, 14, 15, Fig. 185) and the cochlea 

 (16, Fig. 185). The general arrangement of these parts in situ and 

 their relations to the adjacent structures are shown in Fig. 185. Fig- 

 ure 1 88, showing the bony labyrinth isolated, is from a photograph in 



Riadinger's atlas. 



The vestibule is the 

 central chamber of the 

 labyrinth, communicat- 

 ing with the tympanic 

 cavity by the fenestra 

 ovalis, which is closed 

 in the natural state by 

 the base of the stapes. 

 This is the central 

 ovoid opening shown in 

 Fig. 1 88. The inner 

 wall of the vestibule 

 presents a round de- 

 pression, called the 

 fovea hemispherica, per- 



i, the wide canal, the beginning of the spiral canal of the Crated by a number of 

 2, the fenestra rotunda; 3, the second turn of the coch- small foramina throu"h 



which pass nervous fila- 

 ments from the internal 



Fig. 188. The left bony labyrinth of a newborn child, for- 

 ward and outward view, X 4. From a photograph, and slightly 

 reduced (Riidinger). 



cochlea; 



lea ; 4, the final half-turn of the cochlea ; 5, the border of the bony 

 wall of the vestibule, situated between the cochlea and the semi- 

 circular canals ; 6, the superior, or sagittal semicircular canal ; 

 7, the portion of the semicircular canal bent outward; 8, the pos- 

 terior, or transverse semicircular canal; 9, the portion of the auditory meatUS. Be- 

 posterior connected with the superior semicircular canal ; 10, point Vind this deoression is 

 of junction of the superior and the posterior semicircular canals ; 

 ii, the ampulla ossea externa; 12, the horizontal, or external semi- 

 circular canal. The explanation of this figure has been modified 

 and condensed from Riidinger. 



the opening of the 

 aqueduct of the vesti- 

 bule. In the posterior 



wall of the vestibule are five small round openings leading to the semi- 

 circular canals, with a larger opening below leading to the cochlea. 



The general arrangement of the semicircular canals is shown in 

 Fig. 188(6, 7, 8, 9, 10, 11, 12). 



The arrangement of the cochlea the anterior division of the laby- 

 rinth is shown in Fig. 188 (i, 3, 4). This is a spiral canal, about an 

 inch and a half (38.1 millimeters) long, and one-tenth of an inch 

 (2.5 millimeters) wide at its beginning, gradually tapering to the apex, 

 and making, in its course, two and a half turns. Its interior presents a 

 central pillar around which winds a spiral lamina of bone. The fenestra 



