278 



AUDITORY BIOPHYSICS 



Footplate 

 Superior 



Post 



Stapedius 

 pull " 



Pyramid 



Compression 

 member. 

 R 



Inferior 



rotation 



Superior 

 arch of 

 stapes 



Posterior 



0.015 mm 



Vestibule 



Eustachian 

 tube 



Anterior 



The configuration, mass, structure, and size of the stapes are surpris- 

 ingly uniform in adult man. Its average mass was found to be 2.5 mg 

 (Stuhlman [1937]). Its average overall height is about 4.0 mm. The 

 kidney-shaped footplate is 3.2 mm long and has a maximum width of 



1.4 mm through its minor 

 diameter, which lies off cen- 

 ter towards its anterior edge 

 (Fig. VII-14). It has an 

 average area of 3.2 sq mm, 

 confirming Wrightson and 

 Keith's [1918] measurements 

 on European source material. 

 Two hollow U-shaped crura 

 form the arch, the capitulum 

 forming the keystone. The 

 arch is composed of a thick 

 posterior crus (P.C.) the com- 

 pression member, and an an- 

 terior (A.C.) less rugged crus 

 which acts as a tension mem- 

 ber to accommodate displace- 

 ments, of a rolling variety, 

 hinged at the posterior and 

 inferior ends of the footplate. 



The superior arched surface 

 of the stapes forms a slightly 

 elongated parabolic arch; the 

 inferior arch takes the form of 

 a slightly compressed parabolic 

 curve. 



The capitulum is bent down 

 23° and about 10° backward, 

 but the footplate is set at right 

 angles to the feet of the com- 

 posite arch. 



A thrust directed at the 

 capitulum by the lenticular 

 process tilts the footplate diagonally inward, the inferior and posterior 

 edges acting as hinges. This thrust is the compression factor to 

 which the much stronger parabolic arch reacts. On out motion of the 

 lenticular process, a tension is applied to the capitulum and thence to 

 the more fragile superior arch; this force tilts the stapes around the 

 same hinges but outward. 



Basal 

 turn of 

 cochlea 



Fig. VII-14. Semi-diagrammatic view of 

 stapes and footplate where it makes contact 

 with the vestibular fluid. Note shading on 

 footplate to indicate that anterior-superior 

 edge is thickest. Diagrammatic position of 

 stapes set in its oval window. Note position 

 of posterior end of arch in its depressed posi- 

 tion while its anterior end is under tension. 

 The inferior arch is under pressure while its 

 superior or Gothic arch is under tension. The 

 axis of pressure is the broken line; it passes 

 through the center of pressure C. P. in the 

 footplate. Relative tilt of superior arch 

 about 10° posterior. 



