n8o THE EAR. 



state of consciousness is aroused which does correspond with the number 

 of the physical stimuli and with the period of the auditory vibrator. 

 (3) The mass of each vibrator is such that it will be easily set in motion, 

 and after the stimulus has ceased it will readily come to rest. (4) 

 Damping arrangements exist in the ear, so as to quickly extinguish 

 movements of the vibrators. (5) If a simple tone falls on the ear, there 

 is a pendular movement of the base of the stapes, which will affect all the 

 parts, causing them to move ; but any part whose natural period is 

 nearly the same as that of the sound will respond on the principle of 

 sympathetic resonance, a particular nerve filament or nerve filaments 

 will be affected, and a sensation of a tone of a definite pitch will be 

 experienced, thus accounting for the discrimination of pitch. (6) Intensity 

 or loudness will of course depend on the amplitude of movement of the 

 vibrating body, and consequently on the intensity of nerve stimulation. (7) 

 If a compound wave of pressure be communicated by the base of the stapes, 

 it will be resolved into its constituents by the vibrators corresponding 

 to tones existing in it, each picking out its appropriate portion of the 

 wave, and thus irritating corresponding nerve filaments, so that nervous 

 impulses are transmitted to the brain, where they are fused in such a 

 way as to give rise to a sensation of a particular quality or character, but 

 still so imperfectly fused that each constituent, by a strong effort of 

 attention, may be specially recognised. The last statement gives an 

 explanation of the analytic powers of the ear. 



The structure of the ductus cochlearis meets the demands of this 

 theory. It is highly differentiated, and its parts appear suitable for 

 executing independent vibrations. The minute size of the structures 

 does not present any difficulty ; because, however minute the vibrators 

 might be, if they had different periods, they must act in obedience to the 

 same principles of resonance as larger bodies do outside the ear. In 

 1863, Helmholtz was of opinion that the different degrees of tension in 

 the arches of Corti indicated capacity for vibrating at different periods. 

 Soon afterwards it was shown by Hasse x that these rods do not exist in 

 birds, animals presumably capable of appreciating tones ; and Hensen 2 

 pointed out that the membrana basilaris consists of transverse fibres, which 

 vary in length from '04125 mm. at the base of the cochlea to - 495 

 mm. at the hamulus. This led Helmholtz to state that " it is probably 

 the breadth of the membrana basilaris in the cochlea which determines 

 the tuning." 3 He pointed out that the membrane was in a state of con- 

 siderable tension transversely, while it had only little tension in the 

 longitudinal direction, and that such a membrane had very different 

 properties from that of a membrane which had the same tension in all 

 directions. The membrana, from its structure, behaves like a system 

 of stretched strings, bound together by a semifluid substance. Each 

 string or fibre would act independently of the others, and would be set 

 into vibration by an impulse to the fluid in the scala vestibuli, corre- 

 sponding to its period. Consequently, if a part of the membrana were 

 called into action, one of its radial fibres, corresponding to the 

 exciting tone, would vibrate, and the vibrations would extend with 

 diminishing strength on the adjacent portions of the membrane. 

 Possibly some of the structures on the surface of the membrane might 

 act as dampers. In this way the parts of the membrane near the 



1 "De cochlea avium," Kiel, 1866. 



2 Ztschr. f. wissensch. Zool., Leipzig, Bd. xiii. S. 492. 3 Op. cit., p. 218. 



