956 A MANUAL OF PHYSIOLOGY 



portion of the temporal bone, filled with a liquid called the peri- 

 lymph, in which, anchored by strands of connective tissue, floats a 

 corresponding series of membranous canals (the membranous laby- 

 rinth), filled with a liquid called endolymph. The labyrinth of the 

 internal ear is divided into three well-marked parts : the cochlea, 

 the vestibule, and the semicircular canals (Fig. 421). The cochlea, 

 the most anterior of the three, consists of a convoluted tube which 

 coils round a central pillar, the columella or modiolus, like a spiral 

 staircase . The lamina spiralis proj ects from the mod iolus and divides 

 the tube into an upper compartment, the scala vestibuli, and a lower, 

 the scala tympani (Fig. 422) . The part of the lamina next the modi- 

 olus is of bone, but it is completed at its outer edge by a membrane, 

 the lamina spiralis membranacea, or basilar membrane. The scala 

 tympani abuts on the fenestra rotunda, and its perilymph is only 

 separated from the air of the tympanic cavity by the membrane 

 which closes that opening. At the apex of the cochlea the lamina 

 spiralis is incomplete, ending in a crescentic border, so that the scala 



^_ M 



FIG. 421. DIAGRAM OF RIGHT MEMBRANOUS LABYRINTH (AFTER 

 TESTUT). 



i, utricle ; 2, 3, 4, superior, posterior, and* horizontal semicircular canals ; 

 5, saccule ; 6, ductus endolymphaticus arising by two branches, 7, 7' ; 8, saccus 

 endolymphaticus ; g, canalis cochlearis (canal of the cochlea) ending at 9', and 9" ; 

 10, canalis reuniens. 



tympani and the scala vestibuli here communicate by a small 

 opening, the helicotrema. The scala vestibuli communicates with 

 the vestibule, and the vestibule with the semicircular canals, so that 

 the perilymph of the entire labyrinth forms a continuous sheet 

 separated from the cavity of the middle ear by the structures that 

 fill up the round and oval foramina. In the membranous labyrinth, 

 and in it alone, are contained the end-organs of the auditory nerve. 

 The membranous portion of the cochlea is a small canal of triangular 

 section, cut off from the scala vestibuli by the membrane of Reissner, 

 which stretches from near the edge of the bony spiral lamina to the 

 outer wall (Fig. 423), to which it is attached by the spiral ligament. 

 The canal has received the name of the scala media, or canal of the 

 cochlea. The membrane of Reissner forms its roof. Its floor is 

 composed (i) of the projecting edge of the spiral lamina, called the 

 limbus, and (2) of the basilar membrane. The most conspicuous con- 

 stituent of the basilar membrane is a layer of stiff, parallel, trans- 



