322 ELEMENTS OF HISTOLOGY. [Chap. XLI. 



the dura mater, covering the posterior surface of the 

 petrous bone. The saccule is in communication with 

 tne cochlear canal or scala media, by a short narrow 

 tube the caiialis reimiens of Reichert. Thus the 

 cavity of the whole membranous labyrinth is in 

 direct communication throughout all divisions, and it 

 represents the inner lymphatic space of the labyrinth. 

 There is no communication between the perilymph 

 and endolymph, and the cavity of the membranous 

 labyrinth stands in no direct relation to the cavum 

 tympani, since the fenestra ovalis and fenestra rotunda 

 both separate the perilymphatic space, or the cavity 

 of the bony labyrinth, from the cavum tympani. The 

 vibrations of the membrana tympani, transferred by 

 the ossicula auditus to the fenestra ovalis, directly 

 affect, therefore, only the perilymph. The fluctuations 

 of this pass from the vestibule, on the one side, 

 towards and into the perilymph of the semicircular 

 canals ; and on the other side, through the scala 

 vestibuli, to the top of the cochlea, then by the heli- 

 cotrema into the scala tympani, and find their conclu- 

 sion on the membrana secundaria closing the fenestra 

 rotunda. On their way they affect, of course, the 

 membrane of Reissner (see below) separating the scala 

 media from the scala vestibuli ; and the vibrations of 

 this membrane naturally affect the endolymph of the 

 scala media and the terminations of the auditory 

 nerve-fibres (see below). 



463. Structure of semicircular canals, 

 utricle and saccule. The membranous semi- 

 circular canals are fixed by stiff bands of fibrous 

 tissue to the inner periosteum of the one (convex) 

 side of the osseous canal, so that towards the concave 

 side there is left the space for the perilymph. A 

 similar condition obtains with regard to the saccule 

 and utricle, which are fixed by the inner periosteum 

 to one side of the bony part. 



