THE SENSE OF HEARING. 817 



soon learn to know as the scala vestibuli, and the lower chamber as the scala 

 tympani (Fig. 277 ; PI. 2, Fig. 3). The bony shelf mentioned above as partly 

 bisecting the cochlear tube has, of course, like the latter, a spiral course, and is 

 known as the lamina spiralis ; its importance as a supporter of the auditory- 

 nerve filaments will soon be seen. 



Contained within the cavity of the bony labyrinth, and parallel with its walls, 

 is the membranous labyrinth, in which are found the essential structures of the 

 organ of hearing (PI. 2, Fig. 4 ; Fig. 278). The membranous labyrinth is filled 

 with a somewhat watery, mucin-holding fluid, the endolymph, while a similar 

 fluid, the perilymph, is found outside it and within the osseous labyrinth. The 

 perilymph space, which is lined by lymphatic epithelium, is in communication, 

 along the sheath of the auditory nerve, with the subdural and subarachnoid 

 lymph-areas of the brain. Numerous sheets and bars of connective tissue cross 

 from the wall of the bony to that of the membranous labyrinth and help support 

 the latter. That part of the membranous labyrinth lying within the vestibule 

 is composed of two separate sacs a larger posterior, known as the utricle or 

 utriculus, and a smaller, more anterior, known as the saccuk or sacculus. The 

 plane of division between the two sacs ends opposite the fenestra ovalis (PI. 2, 

 Fig. 4). Though the sacs are quite separate, their cavities are indirectly continu- 



9 



FIG. 278. Diagram of right membranous labyrinth seen from the external side (after Testut) : 1, utri- 

 cle ; 2, 3, 4, superior, posterior, and horizontal semicircular canals ; 5, saccule ; 6, ductus endolymphat- 

 icus, with 7, 7', its twigs of origin ; 8, saccus endolymphaticus ; 9, caualis cochlearis, with 9', its vestibular 

 cul-de-sac, and 9", its blind extremity ; 10, canalis reunions. 



ous, through the union of two small tubes arising from either sac, which tubes 

 unite to form the ductus endolymphaticus, a tube running inward through a 

 canal in the petrosal bone and ending blindly in a dilated flattened extremity, 

 the saccus endolymphaticus, this being supported between the layers of the 

 dura mater within the cavity of the skull (PI. 2, Fig. 4). Bundles of audi- 

 tory-nerve fibres penetrate the wall of each sac. The utricle gives rise to the 

 membranous semicircular canals, which communicate with it at five points, 

 it being remembered that the anterior and posterior canals fuse into a single 

 tube at the ends not provided with ampullae, and that they have a common 

 entrance into the utricle. The saccule is continuous by a narrow tube, the 

 canalis reuniens, with that division of the membranous labyrinth contained 



52 



