THE INTERNAL EAR OR LABYRINTH 



The Bony Semicircular Canals (canales semicircular -es ossei). The bony semi- 

 circular canals are three in number, superior, posterior, and lateral, and are situated 

 above and behind the vestibule. They are unequal in length, compressed from side 

 to side, and each describes the greater part of a circle. Each measures about 

 0.8 mm. in diameter, and presents a dilatation at one end, called the ampulla, which 

 measures more than twice the diameter of the tube. They open into the vestibule 

 by five orifices, one of the apertures being common to two of the canals. 



Acoustic 

 nerve' 



\ j\ Plane of superior 



Lt'A / semicircular canal 



T*\ Facial nerve 



\ 



FIG. 922. Position of the right bony labyrinth of the ear in the skull, viewed from above. The temporal bone is con- 

 sidered transparent and the labyrinth drawn in from a corrosion preparation. (Spalteholz.) 



The superior semicircular canal (canalis semicircularis superior), 15 to 20 mm. 

 in length, is vertical in direction, and is placed transversely to the long axis of the 

 petrous portion of the temporal bone, on the anterior surface of which its arch 

 forms a round projection. It describes about two-thirds of a circle. Its lateral 

 extremity is ampullated, and opens into the upper part of the vestibule; the oppo- 

 site end joins with the upper part of the posterior canal to form the eras commune, 

 which opens into the upper nd medial part of the vestibule. 



The posterior semicircular canal (canalis semicircularis posterior), also vertical, is 

 directed backward, nearly parallel to the posterior surface of the petrous bone; 

 it is the longest of the three, measuring from 18 to 22 mm.; its low r er or ampullated 

 end opens into the lower and back part of the vestibule, its upper into the crus 

 commune already mentioned. 



The lateral or horizontal canal (canalis semicircularis lateralis; external semicircular 

 canal) is the shortest of the three. It measures from 12 to 15 mm., and its arch 

 is directed horizontally backward and lateralward ; thus each semicircular canal 

 stands at right angles to the other two. Its ampullated end corresponds to the 



