EXCITATION OF AUDITOR\' RECEPTORS 



569 



.Axis ligaments, 



Ear drum 



Pivot 



FIG. 4. Arrangement of 

 incus and malleus showing 

 how the mass is distributed 

 around the axis of rotation. 

 The maximum displace- 

 ment of the drum occurs 

 at its lower edge. [From 

 von Bekesy (18), after 

 Barany (i).] 



' FIG. 5. The human 

 tympanic membrane turns 

 on an axis near its upper 

 rim. A fold on the lower 

 rim permits movement of 

 the rigid eardrum cone. 

 [From von Bekesy (18).] 



response to mechanical stimulation in the car canal. 

 The refle.x contraction is not very well sustained The 

 contraction does not move the tympanic membrane 

 significantly in man, but the increased stiffness (and 

 perhaps damping also) of the ossicular chain reduces 

 the transmission of low-frequency and of very-high- 

 frequency sounds. The reflex seems to be primarily 

 protective. 



Frequency Characteristics of Ear 



The middle ear has a resonant frequency of vibra- 

 tion of about 1700 cps but its movements are quite 



heavily, although not critically, damped. The reso- 

 nant frequency of the chain of ossicles is raised slightly 

 when the tympanic muscles contract. The external 

 ear canal has a resonant frequency at about 4000 

 cps, which gives an increase of sensitivity of about 10 

 db at this frequency. This resonance combines with 

 that of the middle ear to give an overall acoustic 

 frequency response of the ear that has a broad maxi- 

 mum from 800 to 6000 cps but which falls off rather 

 rapidly above 6000 and, less rapidly, below 800 cps. 

 The main features of the human threshold curve of 

 acoustic sensitivity are apparently determined very 

 largely by these acoustic properties (fig. 6). 



Meclianical Properties oj Inner Ear Structures^ 



In the inner ear the basilar membrane widens 

 gradually from 0.04 mm at the stapes to 0.5 mm at 

 the helicotrema. Certain other measurements, such as 

 cross section of the cochlear canal and relative sizes 

 of certain types of cell in the organ of Corti, are also 

 graded from end to end; but the important variation 

 that allows the cochlea to act as a mechanical acoustic 

 analyzer is in the width of the basilar membrane. As 

 a result of this variation the stiffness ('volume elas- 

 ticity') of the cochlear partition varies by a factor of 

 at least 100 from one end to the other. 



The cochlear partition has significant stiffness and 

 also inass. Contrary to earlier opinions it is not under 

 tension. When cut the edges do not retract. The move- 

 ments of the partition, like those of the middle ear, 

 are quite heavily, but not critically, damped. Because 

 of the gradation in stiffness and mass, different parts 

 of the basilar membrane have different resonant fre- 

 quencies, but the various parts cannot move as inde- 

 pendent resonators. The basilar membrane and the 

 organ of Corti on it are continuous structures. Their 

 elements are coupled to one another elastically and 

 also by friction. The endolymph and the perilymph 

 provide some of the friction. 



Traveling U^ave Pattern nj Cochlear Partition'' 



An increase in pressure on the footplate of the 

 stapes caused by a sound wave sends a wave of 

 acoustic pressure up the cochlea with a velocity that 

 is determined by the laws of transmission of acoustic 



' See especially the papers of von Bekesy (20) and von 

 B6kesy & Rosenblith (22). 



'See especially the papers of Tasaki el at. (17), von Bekesy 

 (20, 21) and von Bekesy & Rosenblith (22). 



