860 THE ORGANS OF SPECIAL SENSE. 



between the parts of the internal ear. It is situated on the inner side of the 

 tympanum, behind the cochlea, and in front of the semicircular canals. It is 

 somewhat ovoidal in shape from before backward, flattened from within outward, 

 and measures about one-fifth of an inch from before backward, as well as from 

 above downward, and about one-eighth of an inch from without inward. On its 

 outer or tympanic wall is the fenestra ovalis, closed, in the recent state, by the base 

 of the stapes, and its annular ligament. On its inner wall, at the fore part, is a 

 small circular depression, fovea hemisphcerica or recessus sph&ricus, which is per- 

 forated, at its anterior and inferior part, by several minute holes (macula cribrosa 

 media) for the passage of filaments of the auditory nerve to the saccule ; and 

 behind this depression is a vertical ridge, the crista vestibuli. This ridge bifurcates 

 below to enclose a small depression, the fossa cochlearis, which is perforated by a 

 number of holes for the passage of filaments of the auditory nerve which supply 

 the posterior end of the ductus cochlearis. At the hinder part of the inner wall is 

 the orifice of the aquceductus vestibuli, which extends to the posterior surface of 

 the petrous portion of the temporal bone. It transmits a small vein, and contains 

 a tubular prolongation of the lining membrane of the vestibule, the ductus endo- 

 lymphaticm, which ends in a cul-de-sac between the layers of the dura mater within 

 the cranial cavity. On the upper watt or roof is a transversely oval depression, 

 fovea semi-elliptica, separated from the fovea hemisphaerica by the crista vestibuli 

 already mentioned. Behind, the semicircular canals open into the vestibule by 

 five orifices. In front is an elliptical opening, which communicates with the scala 

 vestibuli of the cochlea by an orifice, apertura scala' vestibuli cochlea. 



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 is placed trans- 

 versely 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 outer extremity, which is ampullated, communicates by a distinct 

 orifice in the upper part of the vestibule ; the opposite end of the canal, which is 

 not dilated, joins with the corresponding part of the posterior canal to form the 

 cms commune, which opens into the upper and inner part of the vestibule. 



The posterior semicircular canal, also vertical in direction, is directed backward, 

 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, where it opens close to the 

 ampullary end of the superior canal ; 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, with a slight 

 inclination downward, 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 division of the auditory nerve. It measures nearly a quarter of an 

 inch (5 mm.) from base to apex, and its breadth across the base is somewhat greater 

 (about 9 mm.). It consists of a conical-shaped central axis, the modiolus or 

 columella ; of a canal, the inner wall of which is formed by the central axis, 

 wound spirally around it for two turns and three-quarters, from the base to the 



