EAR AS AN ORGAN FOR SOUND SENSATIONS. 



393 



rows, according to the part of the cochlea examined on the outer 

 side of the rods of Corti. Their total number has been estimated 

 differently by different observers; but, accepting the lower figures 

 given, it may be said that there are at least 3500 inner hair cells 

 and 13,000 outer ones, giving a total of 16,500 or more. The theory 

 usually proposed to account for the mechanism by which the vibra- 

 tions of the perilymph affect these cells, and especially the expla- 

 nation of the means by which different sounds affect different cells, 

 is that there is contained in the cochlea a mechanism which acts 

 by sympathetic resonance. To make this theory clear a short 



Fig. 172. Diagrammatic view of the organ of Corti, the sense cells, and the accessory 

 structures of the membranous cochlea (Testut): A, Inner rods of Corti ; B, outer rods 

 of Corti ; C, tunnel of Corti ; D, basilar membrane; E, single row of inner hair (sense) 

 cells; 6, 6', 6", rows of outer hair (sense) cells; 7, 7', supporting cells of Deiters. The 

 ends of the inner hair cells are seen projecting through the openings of the reticulate 

 membrane. The terminal arborizations of the cochlear nerve fibers end around the 

 inner and outer hair cells. 



description must be given of the nature of sound waves and the 

 physical facts in regard to sympathetic resonance. 



The Nature and Action of Sound Waves. Sound waves in 

 air consist of longitudinal vibrations of the air molecules, alternate 

 phases of rarefaction and condensation. For convenience' sake, 

 these waves are usually represented graphically after the manner of 

 water waves, by a curved line rising above and falling below a 

 median zero line, the ordinates above the zero line representing 

 the phase of condensation, and those below the phase of rarefaction. 

 These waves are produced by the vibrations of the sounding body, 

 and may vary greatly in length, in amplitude, and in form. For 

 musical sounds within the range of hearing the length of the waves 

 may vary from forty to seventy feet, at the one extreme, to a frac- 



