EXCITATION OF AUDITORY RECEPTORS 



567 



SCALA TYMPANI 



(PERILTMPh) 



FIG. '2. Cross section of the cochlear partition of the guinea pig in the lower part of the second 

 turn. [From Davis (5).] 



the outer wall of the cochlear canal. The organ of 

 Corti lies on the basilar membrane. The second, ex- 

 ternal, side of the triangle is largely covered by the 

 stria vascularis, so called because it is richly provided 

 with capillaries. This thick layer of specialized cells 

 that face into the cochlear duct is thought to secrete 

 the fluid, the endolymph, that fills the duct. The 

 third side of the cochlear duct, Reissner'.s membrane, 

 is thin but double-layered. It extends from the edge 

 of the stria vascularis acro.ss to the modiolus and 

 separates the space within the cochlear duct, the 

 scala inedia, from the .scala vestibuli. The basilar 

 membrane separates the scala media from the scala 

 tympani. The scala vestibuli and scala tympani are 

 filled with perilymph, a fluid closely resembling cere- 

 brospinal fluid. 



The cochlear partition, including both the basilar 

 membrane and Reissncr's membrane, ends a little 

 short of the apical end of the cochlear canal (fig. 3). 

 Here the scala vestibuli and the scala tympani join 

 through the helicotrema while the scala media ends 

 blindly. At the other end of the scala tympani is the 

 round window. The scala vestibuli opens into the 

 central chamber of the labyrinth, the vestibule, close 

 to the oval window. The length of the cochlear parti- 

 tion in inan, from its origin between the oval and the 



round window to the helicotrema, is about 35 mm. 

 The sensory surface of the cochlea is thus a long 

 narrow ribbon, coiled in spiral form, mounted on an 

 elastic membrane between two fluid-filled channels. 

 This membrane is moved by the fluid which is driven 

 acoustically at the oval window by the last of the 

 ossicles, the stapes. The cochlear partition is the me- 

 chanical frequency analyzer of the ear. 



FUNCTIONAL ANATOMY AND ACOUSTIC 

 PROPERTIES OF EAR 



Onlv those anatomical features of the ear will be 

 described that are necessary for understanding how 

 the ear acts as an acoustic impedance matching sys- 

 tem, an acoustic frequency analyzer and a inechanical 

 stimulator. Anatomy and physiological acoustics will 

 be combined. 



Middle Ear: Acoustic Impedance Malcliing^ 



The tympanic membrane is a light but fairly stiff 

 cone with an apical angle in man of about 135° and 



^ See especially the papers of Stuhlman (13), von Bekesy & 

 Rosenblith (22) and \Ve\er & Lawrence (24). 



