922 



THE ORGANS OF SENSE. 



ispherica by the pyramidal eminence already mentioned. Behind, the semicir- 

 cular canals open into the vestibule by five orifices. In front is a large oval 

 opening which communicates with the scala vestibuli of the cochlea by a single 

 orifice, apertura scalce vestibuli cochlece. 



The Semicircular canals are three bony canals situated above and behind the 

 vestibule. They are of unequal length, compressed from side to side, and describe 

 the greater part of a circle. They measure about one-twentieth of an inch in 

 diameter, and each presents a dilatation at one end, called the ampulla, which 

 measures more than twice the diameter of the tube. These canals open into the 

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



The superior semicircular canal is vertical in direction, and stretches across 

 the petrous portion of the temporal bone, at right angles to its posterior surface ; 

 its arch forms a round projection on the anterior surface of the petrous bone. It 

 describes about two-thirds of a circle. Its outer extremity, which is ampullated, 

 commences by a distinct orifice in the upper part of the vestibule ; the opposite end 



Opening of aqueductm vestibuli. 

 Bristle passed through foramen rotundum. 



Opening of aqueductus cochlese. 

 FIG. 547. The osseous labyrinth laid open. (Enlarged.) 



of the canal, which is not dilated, joins with the corresponding part of the pos- 

 terior canal, and opens by a common orifice with it in the back part of the 

 vestibule. 



The posterior semicircular canal, also vertical in direction, is directed back- 

 ward, nearly parallel to the posterior surface of the petrous bone ; it is the 

 longest of the three : its ampullated end commences at the lower and back 

 part of the vestibule, its opposite end joining to form the common canal already 

 mentioned. 



The external or horizontal canal is the shortest of the three, its arch being 

 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 vestibule, just above the fenestra ovalis; its opposite end opens by a 

 distinct orifice at the upper and back part of the vestibule. 



The Cochlea bears some resemblance to a common snail-shell : it forms the 

 anterior part of the labyrinth, is conical in form, and placed almost horizontally 

 in front of the vestibule ; its apex is directed forward and outward toward the 

 upper and front part of the inner wall of the tympanum ; its base 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 branch ot the 



