HEARING. 



261 



posterior divisions of the labyrinth. The posterior part of 

 the osseous labyrinth is called the osseous vestibule, and 

 contains the membranous vestibule floating within it; the 

 anterior part is called the cochlea, and in it the osseous and 

 membranous parts are more intimately connected one with 

 the other. However, in the dry bone, the parts seen are the 

 cavity of the vestibule, with three semicircular canals coming 

 off from it behind; and, in front of the vestibule, the cochlea, 

 a spiral tube coiled on itself like a snail's shell. 



The cavity of the vestibule is the part into which the fenes- 

 tra ovalis opens. The three semicircular canals spring from 

 its back part; they are about a twentieth of an inch in width, 

 and each makes a circuit about a quarter of an inch in 

 diameter. One of them, the external, is placed horizontally; 

 the other two, called the posterior and the superior, are 

 somewhat larger, and lie in planes at right angles one to the 

 other; the posterior being close to the hinder surface of the 

 bone, and the superior springing from a part which is common 

 to it and the posterior canal, and arching outwards, touching 

 the upper surface of the bone. Each semicircular canal has 

 a dilatation or ampulla near one extremity. 



Fig. 129. MEMBRANOUS LABRYNTH, diagrammatic view, a, 5, c y 

 Superior, posterior, and external semicircular canals ; d, utricle; 

 e, saccule; /, canalis reuniens; g, canalis membranacea cochleae. 



The membranous vestibule, lodged within the osseous, is 

 connected with it merely by vessels and nerves. It presents 

 two cavities separated by a thin partition : the anterior is 

 named the saccule, and connected, as we shall see, with the 

 cochlea; the posterior, which is larger, is called the utricle, 

 and gives off membranous semicircular canals, which lie loose 



