DEVELOPMENT OF THE EAR 717 



into synchronous vibrations by the undulations in the endolymph of the 

 cochlear duct transmitted through the vestibular membrane from the peri- 

 lymph. The latter receives the sound waves through the I'm it-plate of the 

 stapes. 



The Helmholtz (1896) theory of tone perception, until recently widely 

 accepted, postulated sympathetic vibrations, in resonance with atmospheric 

 waves, on the part of the fibers of the basilar membrane. This view has 

 been shown to be untenable, notably by Shambaugh (Archives of Otology, 

 37, 6, 1908) and by Hardesty (Amer. Jour. Anat., 8, 2, 1908). The basilar 

 membrane contains only a little more than half as many fibers as the 

 maximum number of vibrations (40,000 double vibrations per second) com- 

 monly audible; moreover, it does not possess the physical and histologic 

 properties demanded by the 'resonance theory' of tone perception. Ilar- 

 desty has shown that the tectorial membrane on the contrary does answer 

 the requirements of an alternative theory, a modification of the earlier 

 'telephone theory' of Rutherford (1886) ; and he has succeeded in construct- 

 ing an apparatus which simulates the cochlea, and imitates its presumed 

 functional activity at least in the lower ranges of the tone scale. 



Hardesty suggests 'that notes up to a certain pitch throw the entire 

 natural tectorial membrane into vibrations of corresponding frequencies 

 and that sensations of pitch are determined by the frequency of impinge- 

 ment of the membrane upon the auditory hairs, intensity being determined 

 by the amplitude and quality by the quality of the wave motion imparted. 

 Further, that the highest notes within the range of the auditory apparatus 

 throw, according to their frequency, only varying extents of the smaller, 

 basal end of the tectorial membrane into vibration, being so damped out 

 in passing toward the apex of the cochlea, overcoming friction, the inertia 

 of the endolymph and that of the membrane itself, as not to produce vibra- 

 tions in the heavier, apical portions.' (Anat. Rec., 8, 2, 11)14). 



In essence, the tectorial membrane is conceived to respond in its sev- 

 eral parts in the manner of a physical resonator to tones of different pitch 

 (Shambaugh). According to this conception, tone analysis is accomplished 

 peripherally, the specific stimulus being carried to the brain by the special 

 sets of cochlear nerve fibers. 



DEVELOPMENT OF THE EAR 



The external ear develops in connection with the first branchial fur- 

 row: the meatus from the deepened groove, the auricle through the fusion 

 of definite tubercles on the adjacent branchial arches. 



The middle ear and auditory tube arise from the corresponding pharyn- 

 geal pouch, the process involving a ventral elongation of the groove to form 



