THE MEMBRANOUS LABYRINTH. 



645 



¥\<r. 458. 



the jugular fossa, and transmits a small vein. The scala vestihuli is 

 rather narrower than the scala tjmpani in the first turn of the cochlea ; 

 it commences from the cavity of the vestibule, and communicates, as 

 already described, with the scala tympani at the apex of the modiolus. 



■THE MEMBRANOUS LABYRINTH. 



As before stated, within the osseous labyrinth, and separated from 

 its lining membrane by the perilymph, membranous structures exist in 

 which the ultimate ramifica- 

 tions of the auditory nerve 

 are spread. In the vestibule 

 and semicircular canals these 

 structures have a general re- 

 semblance in form to the com- 

 plicated cavity in which they 

 are contained. They do not, 

 however, lie loose within the 

 osseous cavity, as would ap- 

 pear from fig. 458, but are 

 more or less united with its 

 lining periosteum by fibrous 

 bands conveying blood-vessels, 

 which stretch across the space 

 between, and serve thus to fix 

 the membranous sacs and tubes. 

 In the cochlea the membranous 

 structures complete the sep- 

 tum between the scalae already 

 mentioned, and enclose an 

 intermediate passage, the 

 canalis memhranaceus. As 

 before stated, the liquid con- 

 tained within the membranous 

 labyi'inth is distinguished as 

 endohjinph. 



Vestibule. — The 7ncm- 

 hranous vestibule consists of 

 two closely connected sacs, 

 and the parts by which they 

 are united to the membranous 

 semicircular canals and canal 

 of the cochlea. 



The larger of the two sacs, 

 the common si/ms or utricle 

 (fig. 458, i i), is of an oblong 

 form, slightly flattened from 

 without inwards. It islodi^fed 



Fig. 458. — The Interior of the Left Laby- 

 rinth WITH its Membranous Parts and 

 Nerves (from Breschet). Magnified. 



The outer wall of the osseous labyrinth is in 

 jiart removed so as to display the membranous 

 parts within. u, scala vestibuli ; t, scala 

 tympani of the cochlea : s, s, lamina spiralis ; 

 i, i, utriculus or common sinus with its group of 

 otoliths, I: ; I, I, saccule with its otoliths ; a, mem- 

 branous ampulla of the superior semicircular 

 canal, d ; b, ampulla of the horizontal, e, and 

 c, that of the posterior semicircular canal, / ; 

 V, anterior division of the auditory nerve giv- 

 ing branches, q, o, p, to the utricle and the am- 

 pullaj of the superior and external canals ; g, 

 tlie united jjart of the superior and posterior 

 canals ; h, the postei'ior extremity of th© 

 external canal ; * *, space containing perilymph. 



in the upner and back part of 

 the ossco"i'"5 vestibule, occupy- 

 ing the fovea hemi-elliptica. Opposite the crista vestibuli several small 

 branches of the auditory nervo enter from the foramina in the bone ; 

 and here the walls of the common sinus are thicker and more opaque 

 than elsewhere. A small mass of calcareous particles, otoliths or otoconia, 



