820 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of this triangle is attached closely to the bony wall of the cochlea ; the upper 

 side, supposing the modiolus to be vertical with its apex above, is made of a thin 

 sheet of cells known as the membrane of Reissner ; the lower side is made up 

 partly of the bony margin of the lamina spiralis and partly of a membrane, 

 radially striated, stretched across from the edge of the spiral lamina to the side 

 wall of the cochlea ; this is called the basilar membrane, membrana basilaris. 

 The coiled tube forming the bony cochlea is thus divided by the lamina spiralis 

 and the canalis cochlearis into three tubes which wind spirally and parallel 

 round the modiolus. The canalis cochlearis contains endolymph, and its cav- 

 ity ends blindly above and below, but is continuous by way of the narrow 

 canalis reuniens with that of the saccule. The tubes above and below the 

 canalis cochlearis are perilymph-spaces ; it will be noticed that there is no 

 such space on the outer side of the membranous cochlea. 



The upper tube, when followed down to the base of the cochlea, is found 

 to open freely into the vestibule of the labyrinth ; it is therefore known 

 as the scala vestibuli. The lower tube ends blindly at the base of the 

 cochlea, but, where this part bulges into the tympanum as the "promontory" 

 of its inner wall, it is perforated by the aperture known as the fenestra 

 rotunda, whose proper membrane alone prevents the perilymph from escaping 

 into the middle ear. This tube is therefore known as the scala tympani. 

 From its central position the membranous cochlear canal is frequently known 

 as the scala media. The scala vestibuli and the scala tympani both decrease in 

 size as they wind from the base to the apex or cupola of the cochlea ; the 

 membranous cochlear canal, on the contrary, increases in section from base to 

 apex until near the top ; hence the width of the basilar membrane and the 

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

 and the scala tympani have no communication except 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 to 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 scala vestibuli to its top, through the helicotrema, 

 and back by way of the scala 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 



