568 



HANDBOOK (JF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



Perilymph 

 Malleus. Stapes 



Vestibular 

 apparatus 



Scala 

 vestibuli 



External auditory meatus 



Helicotrema 

 Scala tympani 



membrane 



Eustachian^ 

 tube 



FIG. 3. Schematic drawing of tlic liuman car. [From von Bekesy & Rosenblith (22).] 



with flexible edges. It closes the external canal di- 

 agonally. The long process of the first of the ossicles, 

 the malleus, is attached radially to the inner surface 

 of the tympanic membrane from the apex of the cone 

 nearly to its upper edge (figs 4, 5). The similarity of 

 the tympanic membrane to the paper cone of a loud- 

 speaker or microphone is obvious and, like the cone 

 of a microphone, it moves in and out as a whole 

 when driven by sound waves — at least up to about 

 2000 cps. The malleus articulates with the second 

 ossicle, the incus, but, except perhaps at very high 

 intensities, the coupling between them is close and 

 they move as one. The malleus and incus are sus- 

 pended by ligaments in such a way that their only 

 free movement is a rotation around an axis that is 

 nearly tangent to the upper edge of the tympanic 

 membrane. The membrane and the two ossicles turn 

 on this axis as a unit. The rather large heads of 

 malleus and incus serve as a counterweight for their 

 long processes so that the center of gravity of the 

 whole unit is very close to its center of rotation. The 

 system is therefore not readily set in motion relative 

 to the head when the head itself vibrates. This re- 

 duces the sensitivity of the ear to bone-conducted 

 vibration. 



The third ossicle, the stapes, nearly closes the oval 

 window with its "foot-plate," but a flexible annular 

 ligament allows it to swing like a door on an axis that 

 is tangent to the oval window at its posterior end. The 

 long process of the incus articulates with the head of 

 the stapes and drives the latter in a bell-crank type of 

 motion. When the foot-plate of the stapes moves, the 



fluid of the inner ear moves with it. Although the 

 inner ear is a closed chamber, movement is possible 

 because of the yielding of the elastic round window 

 membrane (fig. 3). The latter thus moves in and out 

 in approximately opposite phase to the foot-plate of 

 the stapes. 



The area of the human tympanic membrane is 50 

 to 90 mm^. The area of the foot-plate is about 3.2 

 mm'-. The amplitude of movement of the center of 

 each is about the same. In other words, there is very 

 little mechanical advantage in the lever system in 

 terms of amplitude of movement. The total force at 

 the oval window is about the same as at the tympanic 

 membrane, but it is concentrated in a smaller area; 

 therefore, the pressure exerted on the fluid is greater. 

 The overall system thereby matches the impedance 

 of the air almost exactly to that of the inner ear. As a 

 result, very little acoustic energy is reflected back 

 from the tympanic membrane and nearly all is de- 

 livered to the inner ear. 



Tympiniii Riflrx 



Two small muscles, tensor tsmpani and stapedius, 

 attach to the long process of the malleus and the neck 

 of the stapes, respectively. Each tends to rock its ossi- 

 cle into the cavity of the middle ear. The muscles are 

 thus mechanically antagonistic but they act syner- 

 gistically. They are fast striated muscles and probably 

 not normally in tonic contraction. They do contract 

 reflexly, with a latent period of about 10 msec, in 

 response to fairly strong sounds. They also contract in 



