64 



PHYSIOLOGICAL TRIGGERS 



ments in a short segment of the cochlea that we are concerned. How do they 

 initiate nerve impulses? 



A cross-section diagram of the cochlear canal (iig. 4) shows the structure of 

 the sense organ and allows a visualization of the essential movement. The scala 

 vestibuli and the scala tympani, both filled with perilymph, are separated by 

 the 'cochlear partition.' This partition consists of Reissner's membrane, the 

 tectorial membrane, the organ of Corti and the basilar membrane as flexible or 

 movable parts, and the spiral ligament, the lamina spiralis and the limbus as 



SCALA TYMPANI 



(PERILTMPh) 



Fig. 4. Diagram of the structures of the cochlear partition, including the organ of Corti, 

 based on fixed and stained sections of the second turn of the guinea pig cochlea. Details of the 

 tectorial membrane are based on descriptions of the microdissection of fresh specimens. 



supporting structures. The elasticity of the basilar membrane and the stifTness 

 of the structures attached to it allow it to be displaced in both directions from 

 its position of rest by the pressure of sound waves delivered to the perilymph. 

 The basilar membrane is not under static tension, as was formerly supposed; 

 but the whole structure does have some elasticity, so that it returns, after dis- 

 placement, to a mid position. Parts of it are stififer than others. The rods of 

 Corti are quite stiff and so is the plate-like structure, the reticular lamina, that 

 is supported by the rods of Corti, Deiters' cells and the cells of Hensen. We 

 know, from direct micromanipulation under the microscope (,^) and from the 



