THE EAR AND HEARING. 561 



The Membranous Labyrinth. The membranous vesti- 

 bule, lying in the bony, consists of two sacs communicating 

 by a narrow aperture. The posterior is called the utriculus, 

 and into it the membran- 

 ous semicircular canals 

 open. The anterior, called 

 the sacculus, communi- 

 cates by a tube with the 

 membranous cochlea. The 

 membranous semicircular 

 canals much resemble the 

 bony, and each has an- 

 ampulla; in most of their 

 extent they are only united 



by a few irregular COUneC- . FIG .165. A section through the cochlea 

 J m the hue of its axis. 



tive-tissue bands with the 



periosteum lining the bony canals; but in the ampulla one 

 side of the membranous tube is closely adherent to its bony 

 protector; at this point nerves enter the former. The rela- 

 tions of the membranous to the bony cochlea are more com- 

 plicated. *" A section through this part of the auditory appa- 

 ratus (Fig. 165) shows that its osseous portion consists of a 

 tube wound two and a half times (from left to right in the 

 right ear and vice versa) around a central bony axis, the 

 modiolus. From the axis a shelf, the lamina spiralis, pro- 

 jects and partially subdivides the tube, extending farthest 

 across in its lower coils. Attached to the outer edge of this 

 bony plate is the membranous cochlea (scala media), a tube 

 triangular in cross-section and attached by its base to the 

 outer side of the bony cochlear spiral. The spiral lamina 

 and the membranous cochlea thus subdivide the cavity of the 

 bony tube (Fig. 166) into an upper portion, the scala vesti- 

 buliy &V, and a lower, the scala tympani, ST. Between these 

 lie the lamina spiralis (Isoi) and the membranous cochlea (CO), 

 the latter being bounded above by the membrane of Keissner 

 (R) and below by the basilar membrane (b). The free edge 

 of the lamina spiralis is thickened and covered with con- 

 nective tissue which is hollowed out so as to form a spiral 

 groove (the sulcus spiralis, ss) along the whole length of the 

 membranous cochlea. The latter does not extend to the tip 

 of the bony cochlea; above its apex the scala vestibuli and 

 scala tympani join; both are filled with perilymph, and the 



