376 AN AMERICAS TEXT-BOOK OF PHYSIOLOGY. 



length of its radial fibres increase from below upward. The seala vestibuli 

 and the scala tympani have qo communication excepl through a small aperture 

 under the cupola of the cochlea, known as the helicotrema; this is bounded 

 by the hook-like termination, the hamulus, of the bony lamina spiralis, which 

 forms the greater part of a ring completed by the pointed blind extremity of 

 the canalis cochlearis fastened above it t<> the cupola. 



The Transmission of Vibrations through the Labyrinth. — Vibrations 

 of the tympanic membrane are transmitted as pulses of very small amplitude to 

 the membrane covering the fenestra ovalis. The relatively considerable body of 

 perilymph bathing the inner face of this membrane must be thus set in motion, 

 and there starts a fluid-wave which is free to make its way throughout the 

 perilymph-spaces of the vestibule and the semicircular canals. It may pass 

 from the vestibule along the seala vestibuli to its top, through the helicotrema, 

 and back by way of the seala tympani, at whose bottom it finally surges 

 against the membrane covering the fenestra rotunda; or the wave may be 

 transmitted directly across the membranous cochlea. The fluids of the laby- 

 rinth being physically incompressible, the function of the fenestra rotunda as 

 a sort of safety-valve seems evident. Politzer inserted a glass tube in the 

 round window, and found that fluid in the tube rose when strong air-pressure 

 was brought to bear on the outer side of the tympanic membrane. The cavity 

 of the membranous labyrinth (PI. 1, Fig. I) 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 peri lymph -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 saccnle in turn is 

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



The Membranous Cochlea and the Organ of Corti ( Figs. 193-li)o).— 

 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 <'<>r/i 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 gang/ion spirale. The 



