862 



THE ORGANS OF SPECIAL SENSE.- 



turn a second bony lamina (lamina spiralis secundarid) projects inward from the 

 outer wall of the bony tube ; it does not, however, reach the primary osseous spiral 

 lamina, so that if viewed from the vestibule a narrow fissure, the Jissura vestibuli, 

 is seen between them. 



The Membranous Labyrinth (Fig. 462). 



The membranous labyrinth is contained within the bony cavities just described, 

 having the same general form as the cavities in which it is contained, though 

 considerably smaller, being separated from the bony walls by a quantity of fluid, 

 the perilymph. It does not, however, float loosely in this fluid, but in places is 

 fixed to the walls of the cavity. The membranous sac contains fluid, the 

 endolymph, and on it the ramifications of the auditory nerve are distributed. 



Within the osseous vestibule the membranous labyrinth does not quite preserve 

 the form of the bony cavity, but presents two membranous sacs, the utricle and 

 the saccule. The utricle is the larger of the two, of an oblong form, compressed 

 laterally, and occupies the upper and back part of the vestibule, lying in contact 

 with the fovea semi-elliptica and the part below it. That portion which is lodged 

 in the fovea forms a sort of pouch or cul-de-sac, the floor and anterior wall of which 

 are much thicker than elsewhere, and form the macula acustica utricularis, which 

 receives the utricular filaments of the auditory nerve and has attached to its 



Ductus 

 Endolymphaticus 



FIG. 462. The membranous labyrinth. (Enlarged.) 



internal surface a layer of calcareous particles (otoliths). The cavity cf the utricle 

 communicates behind with the membranous semicircular canals by five orifices. 

 From its anterior wall is given off a small canal, which joins with a canal from the 

 saccule to form the ductus endolympJiaticus. 



The saccule is the smaller of the two vestibular sacs ; it is globular in form, 

 lies in the fovea hemisphaerica near the opening of the scala vestibuli of the 

 cochlea. Its anterior part exhibits an oval thickening, the macula saccularis, to 

 which are distributed the saccular filaments of the auditory nerve. Its cavity 

 does not directly communicate with that of the utricle. From the posterior wall 

 is given off a canal, which joins with a similar canal given off from the utricle to 

 form the ductus endo-lymphaticus. This duct passes along the aqugeductus vesti- 

 buli and ends in a blind pouch on the posterior surface of the petrous portion of 

 the temporal bone, where it is in contact with the dura mater. From the lower 

 part of the saccule a short tube, the canalis reuniens of Hensen, passes down- 

 ward and outward to open into the ductus cochlearis (Fig. 462). 



The membranous semicircular canals are about one-third the diameter of the 

 osseous canals, but in number, shape, and general form they are precisely similar, 



