Semicircular canals

The bony semicircular canals are three in number, anterior (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 

• The anterior (superior) semicircular canal (canalis semicircularis anterior), 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) is vertical and directed backward, nearly parallel to the posterior surface of the petrous bone; it is the longest of the three semicircular canals, measuring from 18 to 22 mm.; its lower end is ampullated (Posterior bony ampulla) and opens into the lower and back part of the vestibule, its upper into the crus commune with the anterior semicircular canal.
• The lateral semicircular canal (horizontal semicircular 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 (lateral bony ampulla) corresponds to the upper and lateral angle of the vestibule, just above the fenestra vestibuli, where it opens close to the ampullated end of the superior canal; its opposite end opens at the upper and back part of the vestibule. The lateral canal of one ear is very nearly in the same plane as that of the other; while the superior canal of one ear is nearly parallel to the posterior canal of the other.