SENSATIONS AND THE SENSE-ORGANS 149 



regarded as the entrance to the labyrinth. The bone 

 is smaller than the opening and the closure is completed 

 by a pliable membrane which plays in and out with the 

 excursions of the ossicle. Beyond this point there is 

 no more air. The vibrations are now represented by 

 the pulsations of a clear fluid, the perilymph. All the 

 numerous and intricate structures within the labyrinth 

 may conceivably be affected by the vibrations introduced 

 into the perilymph from the ossicles but, as has been 

 said, the cochlea is the organ vitally concerned in 

 hearing. 



In a clean dry skull the cochlea is a twisted passage 

 suggestive of the interior of a snail shell, though on a 

 small scale. It makes two and a half turns and its 

 diameter diminishes toward the blind end. Through- 

 out its course it receives, in the living state, nerve 

 fibers to a total number of perhaps 14,000. What is 

 a plain and undivided tunnel in the dried bone is, in 

 life, partitioned into three parts by two membranes 

 which stretch across it. The nerve fibers by means of 

 which we hear originate in certain curious cell-groups 

 associated with one of these, the basilar membrane (mem- 

 brana spiralis of Fig. 37). 



It is supposed that the basilar membrane is shaken 

 in sympathy with the agitation of the perilymph and 

 that stimulation of the endings of the cochlear branch 

 of the auditory nerve is thus brought about. Some 

 physiologists have been led to believe that the vibra- 

 tion rate of the sound waves is reproduced in the rhythm 

 of the impulses which run from the cochlea to the 

 brain. The more common view has been that our sen- 

 sations of pitch are not due to the rhythm of the im- 

 pulses but to the particular fibers which bear them at 

 different times. That part of the basilar membrane 

 which is nearest to the blind end of the cochlea is sup- 

 posed to be more sensitive to slow vibrations, corres- 

 ponding to sounds of low pitch. Proceeding thence 

 along the basilar membrane to the other limit of the 



