THE SENSE OF HEARING. 373 



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 saccule or sacculus. The 

 plane of division between the two sacs ends opposite the fenestra oval is (PI. 1, 

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

 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. 1, 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 ampulla?, 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 

 within the cochlea and known as the canalis cocJdearis. The auditory nerve 

 really consists of two distinct divisions having separate origins and different 

 distributions. One of these branches passes finally to the cochlea, and the 

 other to the vestibule and the semicircular canals. The nerve approaches the 

 labyrinth by way of a canal known as the meatus auditorius internus, and 

 on reaching the angle between the vestibule and the base of the cochlea the 

 cochlear division passes to the cochlea. The remainder of the nerve consists 

 of two divisions, the superior of which is distributed to the utricle and to the 

 ampullae of the anterior and horizontal semicircular canals; the inferior branch 

 supplies the saccule and the posterior semicircular canal. The inner wall of 

 both utricle and saccule is developed at a particular spot into a low mound, 

 the macula acustica, made up of an accumulation of the connective-tissue ele- 

 ments of the membranous wall and covered by a peculiarly modified epithe- 

 lium, the auditory epithelium | Fig. 192). All the auditory-nerve filaments that 

 enter the saccule and utricle respectively pass to these mounds and there enter 

 into relation with the auditory epithelium. 



As the auditory-nerve endings are confined to a particular area in the 

 utricle and the saccule, so the nerve-fibres supplying the semicircular canals 

 are limited to a certain part of the ampulla of each canal. The tissue of the 

 w;ill of the ampulla is developed into a ridge projecting into the cavity in a 

 direction across its long axis. Tins ridge, present in each ampulla, is called 

 the crista acustica; it is capped by a thick layer of columnar epithelial cells, 

 the auditory epithelium, which thins away ;it the border of the crista into 

 the sheet of flattened cells by which the rest of the ampulla is lined. The 

 auditory cells (Fig. 192) are said to be of two kinds — one, cylindrical in 



