140 VETERINARY PHYSIOLOGY 



3. Middle Ear 



The object of the middle ear is to overcome the mechanical 

 difficulty of changing vibrations of air into vibrations of a 

 fluid. It consists of a chamber, the tympanic cavity, placed 

 outside of the petrous part of the temporal bone (fig. 68). Its 

 outer wall is formed by a membrane, the membrana tympani 

 (Ty.), which is attached to a ring of bone. Its inner wall 

 presents two openings into the internal ear the fenestra 

 ovalis (f.o.), an oval opening, situated anteriorly and above, 

 and the fenestra rotunda (f.r.), a round opening placed below 

 and behind. Throughout life these are closed, the former by 

 the foot of the stapes, which is attached to the margin of 

 the hole by a membrane, and the latter by a membrane. The 

 posterior wall shows openings into the mastoid cells, and 

 presents a small bony projection which transmits the stapedius 

 muscle. The anterior wall has above a bony canal carrying 

 the tensor tympani muscle, and below this the canal of the 

 Eustachian tube, which communicates with the posterior nares 

 (fig. 68, -fito.)- 



In the tympanic cavity are three ossicles the malleus (m. ), 

 incus (i.), and stapes (s.), forming a chain between the 

 membrana tympani and the fenestra ovalis. The handle of 

 the malleus is attached to the membrana tympani, and each 

 time a wave of condensation hits the membrane, it drives in 

 the handle of the malleus. This, by a small process, 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. The fenestra rotunda (f.r.) 

 with its membrane acts as a safety valve. The bones rotate 

 round an antero-posterior axis passing through the heads of the 

 malleus and incus. They 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. 



The range of movement is still further controlled by the 

 stapedius muscle, which twists the stapes in the fenestra. This 

 muscle seems to act when loud sounds fall on the ear, and 



