250 THE HUMAN SPECIES 



presented to the waves the better they are picked up; the 

 depth of the funnel aids in this process, so that the human ear, 

 which is scarcely funnel-shaped and quite immobile, fares badly 

 as an acoustic apparatus. 



The vibrations of the tympanum are checked by the ossicles 

 of the middle ear, which form a two-armed unequal lever and 

 act like the damper of a piano. 



The Eustachian tube serves to equalise the pressure of the 

 air in the middle ear and pharynx. 



Tones impinging on the tympanum set up vibrations which 

 are transmitted along the chain of ossicles to the fenestra 

 ovalis, an opening twenty times smaller than the tympanum, 

 so that the amplitude of the vibrations is relatively increased. 

 The orifice of the fenestra ovalis is filled by the foot-plate of 

 the stapes, and when this vibrates the movement is imparted to 



the fluid in the laby- 

 rinth, any excessive 

 vibration of the fluid 

 being in its turn ob- 

 viated by the pres- 

 ence of the fenestra 



rotunda. 

 FIG. 125. Portion of Corti s organ. b, membrana 



basilaris with nerve-fibres (n) ; i, inner ; e, outer The semicircular 

 rods ; i and 2, nuclei of original tissue. , , 



canals containing 



otoliths and hair-like projections are concerned with co-ordinate 

 movements and the balance of the body. 



The cochlea with the organ of Corti is the true nervous 

 receptor for hearing, a microscopical musical instrument, as 

 Lubbock expresses it, upon which the sound waves play like 

 the fingers of the musician on the keys of the piano. 



To every rod-cell a fibre of the auditory nerve is attached ; 

 the appreciation of different tones is due to the stimulation of 

 separate fibres of the auditory nerve which communicate with 

 nerve-cells in the cerebral cortex (in man, apes, dogs and 

 oxen in the temporal lobe). 



Helmholtz's view is that in man the fibres in the organ of 

 Corti are arranged for the seven octaves, and that 33^ fibres 

 correspond with every semitone and 406 with each octave ; 

 although in musical notation only twelve separate notes or 



