THE SENSES. 559 



fenestra ovalis ( 5 ), corresponding in form to the base of the stapes, 

 which nearly fills it, and which is adherent to the internal periosteum 

 of the labyrinth; and 2. The fenes- P IG J-Q 



tra rotunda ( 6 ) of smaller size and 

 closed by a fibrous membrane. The 

 posterior portion of the vestibule 

 gives origin to the three semicircu- 

 lar canals, namely : 1. The superior 

 vertical canal Q with its plane 

 lying across the longitudinal axis of 

 the petrous bone. 2. The inferior 

 vertical canal ( 3 ) the plane of which 

 is parallel with the median surface 

 of the petrous bone : and 3. The _ 



1 * BONY LABYRINTH OF THE HUMAN EAR, twice 



horizontal Canal ( 4 ) lying acrOSS the the natural size. 1. Vestibule. 2. Superior 



axis of the petrous bone, in a hori- Z%^ 



ZOntal plane. Each Semicircular Ca- circular canal. 5. Fenestra ovalis. 6. Fenes- 



nal opens into the vestibule by two trarotunda - 7 - Cochlea, 

 orifices, one at each end ; except that the two vertical canals unite at 

 one extremity into an orifice common to both. Each canal is enlarged 

 at one extremity, where it joins the vestibule, into a rounded dilatation. 



This part of the bony labyrinth contains a colorless fluid the peri- 

 lymph, and, in addition, a membranous sac, also filled with fluid, which, 

 by its prolongations, repeats the form of the vestibule and semicircular 

 canals. This sac, with its extension in the cochlea, constitutes the 

 membranous labyrinth. It forms the most important part of the in- 

 ternal ear, since in its walls the filaments of the auditory nerve have 

 their terminal distribution. 



In the vestibule the membranous sac is divided into two parts by a 

 transverse partition. One of these, the smaller of the two, is the sac- 

 culus, a spherical vesicle, a little over 1.5 millimetre in diameter, occu- 

 pying the anterior and inferior portion of the vestibule, and communi- 

 cating 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 1 diameter. The utricle and the membranous 

 semicircular canals communicate with each other in the same way as 

 the bony cavities in which they are lodged ; and each membranous 

 canal presents, at one extremity, a rounded dilatation, known as the 

 "ampulla.' 1 



The membranous sacs and canals are considerably smaller than the 

 osseous cavities which contain them, and occupy nearly everywhere 

 an eccentric position, being, at certain points, adherent to the internal 

 periosteum, while at others they are surrounded by the perilymph. 

 The sacculus and utricle together occupy about two-thirds of the cav- 

 ity of the vestibule ; and, according to Riidinger, are so placed that 

 neither touches the base of the stapes at the fenestra ovalis, from which 

 they are separated by an appreciable layer of fluid. Thus sonorous im- 



