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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 

The Bony Semicircular Canals (canales semicirculares ossei).—The bony semicircular 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.


FIG. 922– Position of the right bony labyrinth of the ear in the skull, viewed from above. The temporal bone is considered transparent and the labyrinth drawn in from a corrosion preparation. (Spalteholz.) (See enlarged image)

  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 opposite end joins with the upper part of the posterior canal to form the crus commune, which opens into the upper and 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 lower 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

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