EAR AS AN ORGAN FOR SOUND SENSATIONS. 399 



of Corti and of the tectorial membrane they give rise to a trans- 

 verse or shding movement between the upper surface of the organ 

 of Corti and the lower surface of the tectorial membrane so that 

 the cilia of the hair cells are bent or put under a stress (Fig. 1 78a, B). 

 In this way the hair cells and through them the nerve fibers are 

 stimulated. It is assumed that each sound wave will cause four 

 such stimuh, one corresponding to the crest of the wave, one to the 

 trough of the wave, and one for each point where the curve crosses 

 the zero or base-line, as indicated in Fig. 178a. Wrightson states 

 that with four such stimuli the brain is furnished with data sufficient 

 for the analysis of any compound wave into its component primary 

 vibrations. The several structures of the organ of Corti are ex- 

 plained most ingeniously as mechanisms adapted to work in the 

 way required by the theory. Unfortunately, we have no direct 



Phase y / / / 



^"^ \ ■ ■ ■ \ \' \ = -Tecfop/d/ memb. 



A B 



Fig. 178a. — Diagrams to illustrate Wrightson's Theon,- of Hearing (after KeitJi): A, Con- 

 ventional figure of sound wave to illustrate the four phases — the crest, the trough, and the two 

 points at which the curve crosses the zero line. B, Schema to illustrate the effect upon the 

 cilia of the hair cells of the displacement of the basilar membrane by the sound wave: Phase 

 I, displacement downward, giving flexion of cilia inward because of transverse movement be-- 

 tween the tectorial and reticular membranes; Phase III, displacement upward; Phases II and 

 IV, resting position, where the curve crosses the zero line. 



experimental evidence in favor of any of these views. Several ob- 

 servers, however, have demonstrated apparently that, whatever 

 may be the mechanism that is stimulated, it is so arranged that at 

 the base of the cochlea the higher notes are received and at the apex 

 the notes of the lowest pitch. Thus, Munk, in experiments upon 

 dogs, in which by an operation through the fenestra rotunda he had 

 destroyed the basal portion of the cochlea, found that the animals, 

 after a temporary deafness of some days, could hear apparently 

 only low tones and noises. Baginsky,* in a later series of experi- 

 ments, opened the bulla ossea on each side, destroyed the cochlea 

 on one side entirely so as to render that ear deaf, while on the other 

 he injured it in certain areas only. He found that when the apex 

 of the cochlea was destroyed the animal appeared to perceive only 

 the high tones, c'", c"", c'"". 



* Baginsky, "Virchow's Archiv f. pathol. Anat.," 94, 65, 188-3. 



