494 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1924 



the mechanical transmission chain for carrying sound vibrations 

 from the ear drum to a small annular membrane, the fenestra ovalis 

 or oval window. The middle ear also contains the muscles which 

 condition the drum and transmission chain so as to accommodate 

 the mechanism to hearing under the variety of actual conditions. 



The inner ear is a spiral space in the bony shell called the cochlea. 

 This space is filled with fluid. It is separated into two compartments 

 by the narrow flexible basilar membrane except at the apex of 

 the cochlea where a tiny passage, the helicotrema, connects the two 

 compartments. At the base of the cochlea there is a membraneous 

 diaphragm, commonly called the round window, located on the other 

 side of the basilar membrane from the oval window. Within the 

 spiral casing and terminating on the dividing membrane is the 

 multitude of terminals which connect with the hearing center of 

 the brain through the auditory nerve. 



Sound vibrations are transmitted by the stapes through the oval 

 window to the inner ear. At ordinary frequencies vibrations are 

 transmitted through the fluid to a proper distance along the basilar 

 membrane (the appropriate position depending upon the frequency) 

 where they are passed through the membrane and sensed. The 

 excess of vibratory energy transmitted to the second compartment 

 is relieved by the flexibility of the round window. The pitch of 

 a simple tone depends upon the position of maximum response 

 of the basilar membrane — high tones near the base, low tones near 

 the apex of the cochlea. The brain is believed to detect the pitch 

 by its experience in associating tones of different pitches with the 

 stimulation of different nerve groups. When the pitch of the tone 

 is very low, the fluid is moved back and forth around the basilar 

 membrane through the helicotrema. Such impulses follow each 

 other so slowly that the stimulation of the nerve fibers thereby 

 produced is not of the type commonly recognized as a sound sensa- 

 tion. If the pitch of the tone is sufficiently high, the vibratory 

 impedance of the ear mechanism is such that little or no energy is 

 communicated to the inner ear, and in that event also the nerve 

 terminals are unaffected. 



The transmission efficiency of the mechanical system linking 

 the ear drum with the basilar membrane is not equal at all fre- 

 quencies, and its operation varies also with the intensity or loud- 

 ness of tone. Changes of intensity are probably detected either 

 by change in the amount of agitation of the nerve terminals or 

 by bringing into play wider zones of nerve terminals in the vicinity 

 of the greatest vibration. The marvelous delicacy of the ear mecha- 

 nism is called to attention when one considers that, in the average 

 case, the basilar membrane by means of which all of these various 

 tones are sensed is only a little over an inch long. 



