510 Professor Arthur Keith [March 14, 



wall. Thus in the utricular system the hairlets or levers are worked 

 hy gravity : in the canalicular system, mass displacements of fluid 

 set up by mcvements of the head bend the levers and give rise to 

 signals ; in the cochlea the force employed in working the lever 

 system is the minute displacements set up by sound waves, and the 

 levers are bent by the field of hair- cells working against the titillator 

 or tectorial membrane. 



The essential modifications, required to make the otic primitive 

 vesicle into an organ of hearing, were a closed vesicle, filled with 

 fluid and everywhere surrounded by bone of a peculiarly dense 

 nature — all except at one area — where a minute window, the 

 fenestra rotunda, was established. This window is essential, for 

 without it there can be no mass displacement of the fluid and no 

 production of nerve-stimuli as sound waves sweep through the ])ony 

 walls of the vesicle. In the passage leading to the round window is 

 placed the organ of Corti — the apparatus for recording the displace- 

 ments of fluid set up by the bone-conducted sound waves. To make 

 the ear a more sensitive machine another window has been estabhshed 

 in the bony wall of the vesicle— the fenestra ovalis, into which is 

 fixed a movable piston, the stapes. By a bent lever, formed by the 

 ossicles of the ear, this piston is yoked to the membrana tympani, 

 and thus the ear is rendered infinitely more sensitive to sound 

 impulses carried by the air. The area of the drum is fifteen times 

 that of the foot-plate or piston of the stapes. Closure of the 

 fenestra ovalis, by fixation of the stapes, renders the ear more 

 sensitive to bone-conducted waves ; closure of the fenestra rotunda 

 produces complete deafness. These facts cannot be explained on the 

 hypothesis put forward by Helmholtz, but find a complete answer 

 in the theory put forward by Sir Thomas Wrightson. 



Four phases are to be recognized in the completed movement of 

 the lever or hairlet of a sense-cell. Its upright or vertical position 

 may be regarded as one of rest — its zero position. In the first 

 phase of a complete movement the hairlet bends towards one side — 

 towards the right we shall suppose ; in the second it returns to its 

 upright or zero position ; in the third it bends towards the left ; in 

 the fourth it again returns to its starting or zero point. It is clear 

 that different conditions of tensions and pressures will be set up 

 within the hair-cell in each of these four phases, and each phase we 

 may postulate gives rise to a nerve impulse or signal ; the signals set 

 up will vary with the duration and force of each hairlet movement. 

 In each sound wave Sir Thomas Wrightson recognizes four correspond- 

 ing phases : two of these lie in the part of the wave where the air 

 particles are being condensed — the part in which there is a plus 

 pressure ; two of them lie in the part where the air particles are being 

 rarefied — where there is a minus pressure. In phase I the plus 

 pressure is rising ; in phase II the plus pressure is falling ; in phase 

 III the minus pressure is increasing ; in phase IV the minus pressure 



