168 VETERINARY PHYSIOLOGY 



brana tympani and the fenestra ovalis. The handle of the 

 malleus is attached to the membrana tympani, and each time 

 a wave of condensation falls upon the membrane it drives 

 this inwards, and with it the handle of the malleus. This 

 locks the malleo-incal joint (fig. 83, m.i.), and pushes inwards 

 the long process of the incus, which thrusts the stapes into 

 the fenestra ovalis, and thus increases the pressure in the 

 enclosed fluid of the internal ear. By the " give " of the 

 membrane in the fenestra rotunda, the increased pressure is 

 relieved. The bones rotate round an antero-posterior axis 

 passing through the heads of the malleus and incus. They 



ZkM 



ZmT. 



Fig. 83. — Diagram of the Ear. Ex.M., external meatus; Ty., tympanic 

 membrane; m., malleus; i., incus; s., stapes; /.o. , fenestra ovalis; 

 f.r., fenestra rotunda; En.T., Eustachian tube; v., vestibule with 

 the utricle and saccule ; s.c, semicircular canal ; Coch., cochlea. 



thus form a lever with the arm to which the power is applied 

 — the handle of the malleus — longer than the other arm. 

 The advantage of this is that, while the range of movement 

 of the stapes in the fenestra ovalis is reduced, its force is 

 proportionately increased. A further increase of force is 

 gained by the small size of the fenestra ovalis as compared 

 with the tympanic membrane. 



On account of the fact that the annular lisfament is 

 narrow on one side and broader at the other, the movement 

 of the foot of the stapes is not a straight thrust but a move- 

 ment round a hinge. 



