No. I.] THE VERTEBRATE EAR. 269 



The cochlea, on the other hand, thanks to its more differen- 

 tiated and comphcated structure, is able to separate a musical 

 picture into its several parts, to perceive the tonal differences, 

 such as pitch and timbre, and it is probably the place where 

 rustling sounds are distinguished. 



These functions are performed by the following parts. In 

 the utriculo-sacculus and ampullae, by the auditory hairs and 

 the parts they are supported by. In the cochlea, by the asso- 

 ciated fibres of the basilar membrane and hair-bearing cells 

 and arches of Corti, the percipient elements being the hair 

 cells which are directly continuous with the nerves. The indis- 

 pensable sensitiveness or delicacy of perception in the cochlea 

 finds a suitable physical basis in the capsules of Hensen, which, 

 like sense buds, represent organs which are sensitive in the 

 highest degree to the purely mechanical stimuli of sound waves. 



A special analysis of the other parts of the cochlea shows us 

 that in the basilar membrane, from its position, its character, 

 and composition, — being made of straight, elastic fibres some- 

 what like strings tensely drawn between the two scalae, — we 

 have a vibration mechanism, which, as Helmholtz has already 

 shown, is highly capable of vibrating. The vibrations of its 

 fibres must be very regular, though naturally in different parts 

 of its course they would be of varying intensity — that portion 

 on which the hair cells lie being the most active in its move- 

 ments, i.e. having the greatest amplitude, since this part is least 

 loaded and has freest motion. This motion is sufficient to com- 

 municate to the hair cells the requisite stimuli for hearing. 

 The abundance of basilar fibres, of which five hundred are 

 allotted to a half-tone in the human ear, compels us to assume 

 that they fall naturally into smaller or larger groups. This, as 

 we know, agrees with their anatomical relations, which are re- 

 sponsive to various tones, the lowest of which are found near 

 the apex of the cochlea, while the highest tones are perceived 

 at the base. This calculation is based solely upon their length, 

 their varying thickness and the amount of load in the shape of 

 surrounding tissue carried being left out of consideration. 



However these things may be, we have in the cochlea, as 

 everything leads us to believe, the basilar membrane as the 

 vibration apparatus demanded by physiological acoustics, and the 

 hair cells with their capsules as the percipient elements. 



