CH. LIT.] 



MECHANISM OF THE TYMPANUM 



747 



interchange of gases takes place between the imprisoned air and the 

 blood of the tympanic vessels. In time, as in the aerotomometer 

 (see p. 381), equilibrium is established and the tension of the 

 imprisoned gases becomes equal to that of the blood-gases, not to 

 that of the atmosphere. The membrane is therefore cupped inwards 

 by the atmospheric pressure^ on its exterior; it is this increased 

 tightening of the membrane that produces deafness. There is also 

 an accumulation of mucus. When one makes a violent expiration, 

 as in sneezing, some air is often forced through the Eustachian tube 

 into the tympanum. The ears feel as though they were bulged out, 

 as indeed the membrana tympani is, and there is again partial deaf- 

 ness, which sensations are at once relieved by swallowing so as to 

 open the Eustachian tube and thus re-establish equality of pressure 

 once more. 



The ossicles communicate the vibrations of the membrana 

 tympani (to which the handle of 

 the malleus is fixed) to the mem- 

 brane which closes the fenestra 

 ovalis (to which the foot of the 

 stapes is attached). Thus the 

 vibrations are communicated to 

 the fluid of the internal ear which 

 is situated on the other side of 

 the oval window. 



The accompanying diagram will 

 assist us in understanding how 

 this is brought about. The bones 

 all vibrate as if they were one, 

 the slight movements between the 

 individual bones being inappreci- 

 able. The utility of there being several bones is seen when the 

 vibrations are excessive; the small amount of "give" at the 

 articulations is really protective and tends to prevent fractures. 



The handle of the malleus is inserted between the layers of the 

 tympanic membrane ; the processus gracilis (p. g.) has its end A 

 attached to the tympanic wall on the inner aspect of the Glaserian 

 tissure ; the end B of the short process (s. p.) of the incus is fastened 

 by a ligament to the opposite wall of the tympanic cavity ; the end 

 D of the long process of the incus articulates with the stirrup, the 

 base of which is turned towards the reader. The handle vibrates 

 with the membrana tympani ; and the vibrations of the whole chain 

 take place round the axis of rotation AB. Every time C comes 

 forwards D comes forwards, but by drawing perpendiculars from C 

 and D to the axis of rotation, it is found that D is about f of the 

 distance from the axis that C is. So in the transmission of the 



'oot of 

 Stapes 



FIG. 549. Diagrammatic view of ear ossicles. 



