STRUCTURE OF THE LABYRINTH 



233 



At these places the lining epithelium is specially modified to form 

 a sensory or nerve -epithelium ; elsewhere it is a simple pavement- 

 epithelium. 



FIG. 269. PLAX OF THE RIGHT MEM- 

 BRANOUS LABYRINTH VIKWED FROM 

 THE MESIAL, ASPECT. 2j 



u, utricle, with its macula and the three 

 semicircular canals with their ampullae ; 

 t, saccule ; aq. v. aqu.-ednctus vestibu'i ; s. e. 

 saccus endolymphaticus ; c. r. canal is re- 

 uniens ; c. c. canal of the cochlea. 



FIG. 270. VIEW OF THE INTERIOR OF 

 THE LEFT OSSEOUS LABYRINTH. 2} 



The bony wall of the labyrinth is removed 

 superiorly and externally. 1, fovea henii- 

 elliptica ; 2, fovea hemi-phaerica ; 3, common 

 opening 6f the superior and posterior semi- 

 circular canal? ; 4, opening of the aqueduct 

 of the vestibule ; 5, the superior, 6, the 

 posterior, and 7, the external semicircular 

 canals ; 8, spiral tube of the cochlea ; 9, 

 scala tympani ; 10, seal a vestibuli. 



The membranous semicircular canals and the utricle and saccule 

 are composed of fibrous tissue, which is adherent along one side to the 

 endosteum of the bony canal ; from the opposite side bands of fibrous 

 tissue pass across the perilymph. Within the fibrous membrane is a 

 thick clear tunica propria, which, in the semicircular canals, forms 

 papillary elevations in the interior of the tube (figs. 271, 272). 



The places of entrance of the nerve-fibres into the ampulla?, are 

 marked by a transverse, inwardly projecting ridge (crista], in the 

 saccule and utricle by a thickening of the tunica propria {macula}. 

 The epithelium at these places is formed of columnar cells (fig. 273), 

 which are surmounted by long, stiff, tapering hairs (auditory hairs, 

 fig. 273, Ji), and to these hair-cells the axis-cylinders of the nerve-fibres 

 pass directly (fig. 274) ; they are therefore like the rod- and cone- 

 elements of the retina, the bipolar cells of the olfactory membrane, and 

 the gustatory cells of the taste-buds sensory or neural epithelium-cells. 

 Between them are a number of thin and somewhat rigid nucleated 

 cells (fibre-cells of Retzius, fig. 274, /), which rest upon the basement- 

 membrane, and are connected at their free extremity with a cuticular 

 membrane, through which the auditory hairs project. 



The auditory hairs do not project free into the endolymph, but 

 into a soft mucus-like substance, of a dome-like form, in the ampullae 

 (fig. 273), and which in the saccule and utricle has a mass of calcareous 

 particles (otoliths) embedded in it. 



