THE MIDDLE EAR. 1153 



per second of 2560, and Konig places the pitch of the proper tone two 

 octaves higher, namely at mi e , the frequency of which is 10,240. But it 

 is very doubtful if the membrane can be said to have any proper tone 

 in the living body ; if it has such, the pitch is in all probability nearer 

 that assigned to it by Konig than by Politzer. 



The tension of the drum-head is maintained in the living body by 

 the tensor tympani: like other muscles, it possesses a constant tonicity. 

 As the tendon of the muscle is inserted into the handle of the hammer 

 a short distance below the horizontal axis of rotation of the bone, it 

 is obvious that it must exert a constant force tending to draw the mem- 

 brane inwards. 



The cone which is formed by the inwardly drawn membrane has 

 its walls convex outwardly. This is due to the arrangement and 

 relative length and strength of the circular fibres. This mechanism 

 may be illustrated by an experiment upon a spider's web, which is also 

 composed of circular and radiating fibres. If the centre of the web be 

 drawn out at right angles to its plane, it will be found that the sides of 

 the cone thus formed are convex in a direction opposite to that towards 

 which the apex of the cone points (Gray). 



Secchi 1 maintains, as the result of experiment, that there exists in the living 

 subject a pressure in the tympanic cavity of 4 mm. above that of the external 

 air. This statement has neither been confirmed nor denied by other investi- 

 gators, but it is difficult to see how it can be brought into line with well- 

 ascertained facts regarding the tympanum. 



Movements of the membrana tympani in response to sound 

 pressures.— If the drum-head had been a uniformly flat stretched mem- 

 brane, the amplitude of its movements in response to the varying 

 pressures of sound-waves would be greatest in the centre, while it would 

 diminish as the periphery of the membrane was approached. A variety 

 of conditions combine to render the movement much more complex. 

 These are the peculiar curvature of the mem- 

 brane, the fact that it is weighted by the 

 hammer, and the pull which the tensor tym- 

 pani exerts upon it through the hammer. 

 Helmholtz 2 was the first to investigate this T 



, , . . . . , b Fig. 419.— a, Outer margin of 



problem m a satisfactory manner. meatus; b, c, tympanic ring 



As the outward curvature of the radial with the membrane between, 



fibres is slight, each fibre may be regarded as ^ v z ing its curvatures. -Helm- 

 the long arm of a lever, while the handle of the 



hammer is the short arm. This mechanism secures that a slight pressure 

 of the air, corresponding to a sound-wave, exerts a considerable force upon 

 the malleus. To aid in understanding the mechanism, it will be easier to 

 consider, first, the effect of pressure upon a single radial fibre. The fibre 

 may be regarded as inextensible and slightly curved outwards ; hence 

 variations in pressure on the convexity of the curve will cause the 

 degree of curvature to change, while the length of the arc will remain 

 the same. 



1 Trans. Internat. Med. Cong., Berlin, 1891 ; Ro,me, 1894. 



2 "Ueber die Mechanik der Gehorknochelchen," " Wissensehaftliehe Abhandlungen," 

 i. ii. S. 503 and 515; "Die Mechanik der Gehorknochelchen und des Trommel- 

 11s," Arch. f. d. yes. Physiol., Bonn, 1868, Bd. i. S. 1 ; see also the translation of the 

 tter work by A. H. Buck and N. Smith, New York, 1873. The description is taken 



TT _1. 1 Ii J i 



vol. ii. — 73 



