THE SENSE OF HEARING. 821 



of the membranous labyrinth (PI. 2, Fig. 4) is nowhere in communication with 

 the perilymph-space about it, and we must therefore assume that the irritation 

 of the auditory cells seated in its wall must depend on vibrations transmitted 

 from the perilymph directly through the membranous sacs and tubes. 



Like the perilymph-space, the cavity of the membranous labyrinth is in 

 communication throughout, though in certain situations the connection of 

 adjacent parts is very indirect. Thus, though the semicircular canals open 

 freely at both ends into the utricle, the utricle and saccule are only brought 

 into union by the two narrow tubes that unite to form the ductus endolym- 

 phaticus. It will be noted that by means of this duct the membranous laby- 

 rinth is really continued into the cranial cavity. The saccule in turn is 

 continuous with the scala media of the cochlea by way of the canalis reuniens. 



The Membranous Cochlea and the Organ of Corti (Figs. 280-282). 

 The cochlear division of the auditory nerve, together with the nutrient blood- 

 vessels, penetrates the modiolus at its base and runs up through the spongy 

 interior of the bony pillar. As the nerve ascends through the modiolus its 

 fibres are gradually all diverted to run in a radial direction between the bony 

 plates of the lamina spiralis, to terminate in the organ of Corti of the canalis 

 cochlearis. A collection of nerve-cells is interposed in the course of the audi- 

 tory fibres at the base of the lamina spiralis. 



A complete view of the nerves of the cochlea would show a central pillar 

 of nerve-fibres diminishing in thickness from below upward, and winding 

 round this pillar a spiral sheet of radially-disposed nerve-fibres containing, 

 near their point of departure from the central pillar, a spiral line of ganglion- 

 cells ; this collection of cells is therefore known as the ganglion spirale. The 

 thin, free edge of the bony lamina spiralis is, in the recent state, thickened by 

 a development of connective tissue forming a promontory known as the limbus. 

 The free edge of the limbus is in turn shaped in such a way as to make a short, 

 sharp projection in the plane of the upper surface of the lamina and a longer 

 projection in the plane of its lower surface, leaving the free margin between 

 them hollowed out. The upper projection, which is known as the vestibular 

 lip, labium vestibulare, serves for the attachment of the tectorial membrane, 

 membrana tectoria, presently to be described. The lower projection is called 

 the tympanic lip (labium tympanicum) ; to it is attached the inner margin of 

 the basilar membrane, on whose inner half is seated the very complex struct- 

 ure known as the organ of Corti. 



The basilar membrane is a thin sheet of fibrillated connective tissue stretched 

 tightly between the tympanic lip of the limbus on the inside and the spiral 

 ligament (see p. 824) on the outside. The more median part of the membrane, 

 which supports the organ of Corti, is thin and rigid and is fibrillated in a 

 radial direction. The outer part, which is first thicker and then thinner again 

 near its point of attachment, is distinctly composed of radial fibres cemented 

 together; the isolated fibres are characterized by being stiif and brittle. 



The organ of Corti (Figs. 280, 281) has as its supporting basis a series of 

 peculiarly modified epithelial cells, known as the rods of Corti (Fig. 282, B, B'), 



