SENSE OF HEARING. 655 



the semicircular canals and cochlea Fig. 211. 



diverge in various directions. It is 

 of a more or less ovoid form, and 

 presents, toward the cavity of the 

 tympanum, two openings, namely : 

 1. The fenestra ovalis (5), corre- 

 sponding in form to the base of the 

 stapes, which nearly fills it, and 

 which is adherent to the internal 

 periosteum of the labyrinth stretch- 

 ed across the opening ; and 2. The 

 fenestra rotunda (e) of smaller size BoNY LABYKINTH OF THE HUMAN 



EAR, twice the natural size. 1. Vestibule. 



and Closed Only by a fibrOUfl mem- 2 Superior vertical semicircular canal. 3. 

 brane. The posterior portion of the Inferior vertical semicircular canal. 4. 

 .. . . Horizontal semicircular canal. 5. Fenestra 



Vestibule gives origin to the three O valis. 6. Fenestra rotunda. 7. Cochlea. 



semicircular canals, which commu- 

 nicate with its cavity at each extremity, namely: 1. The superior ver- 

 tical canal ( 2 ) the plane "of which is directed across the longitudinal 

 axis of the petrous bone. 2. The inferior vertical canal ( 3 ) the plane 

 of which is parallel with the internal surface of the petrous bone ; and 

 3. The horizontal canal (4) which is directed across the axis of the 

 petrous bone, but lies, as its name indicates, in a horizontal plane. Each 

 semicircular canal opens into the vestibule by two orifices, one at each 

 end ; except that the two vertical canals unite at one of their extremi- 

 ties into a branch and orifice common to both. Thus there are five 

 orifices leading from the vestibule into the three semicircular canals ; 

 and each canal has free communication at each end, directly or indi- 

 rectly with the interior of the vestibule. Each canal is enlarged at one 

 of its extremities where it joins the vestibule, into a slightly rounded 

 dilatation. 



The common cavity of this part of the bony labyrinth contains a 

 limpid, colorless fluid, the perilymph, and, in addition, a closed mem- 

 branous sac, also filled with fluid, which, by its various prolongations, 

 presents a repetition of the form of the vestibule and semicircular canals. 

 This, together with its extension in the cochlea hereafter to be described, 

 constitutes the membranous labyrinth. It forms the most important 

 part of the internal ear, since in its walls the filaments of the auditory 

 nerve have their terminal distribution. 



The cavity of the vestibule contains two membranous sacs, lying in 

 contact with each other, but separated by a transverse partition. One 

 of these, the smaller of the two, is the sacculus, a spherical vesicle, a 

 little over 1.5 millimetre in diameter, occupying the anterior and inferior 

 portion of the vestibule, and communicating by a narrow canal with the 

 ductus cochlearis of the cochlea. The other, or larger sac, is the utricle, 

 of ellipsoid form, measuring 3.5 millimetres in its long diameter. The 

 utricle nnd the three membranous semicircular canals communicate with 

 each other in the same way as the corresponding bony cavities in which 



