1026 



THE SENSES 



movement occurs. Its function is to keep the pressure in the middle 

 ear approximately that of the atmosphere. In a balloon ascent an 

 excess of pressure is established on the internal surface of the tympanic 

 membrane. In the air-lock of a caisson when the air is being com- 

 pressed the excess of pressure is on the external surface of the membrane. 

 The feeling of uncomfortable tension is relieved in both cases by swallow- 

 ing movements which allow the pressure in the tympanum to adjust 

 itself to that in the pharynx. In catarrh of the naso-pharynx the 

 orifice may be occluded, and this is accompanied by impairment of 

 hearing and a disagreeable sensation of tension in the ear, owing to 

 absorption and consequent rarefaction of the air in the tympanum. 



The patient instinc- 

 tively makes efforts 

 which increase the pha- 

 ryngeal pressure from 

 time to time so as to 

 open the tube. 



The loosely - jointed 

 chain of ossicles is 

 steadied and its move- 



ments directed by liga- 



ft -^/tygi^Mg * ments and by the ten- 



sion of its terminal mem- 

 branes. It forms a kind 

 of bent lever by which 

 the oscillations of the 

 membrana tympani are 

 transferred to the mem- 

 brane covering the oval 

 foramen, and at the 

 same time reduced in 

 size. Two slender 

 muscles, the tensor tym- 

 pani and stapedius, con- 

 tained in the tympanic 

 Fig- 435- Tympanum of Left Ear, showing the cavitv are also con- 

 Ossicles (Morris), i, superior, and 4, external, nec "ted with and may 

 ligament of malleus; 2, head; 7, short process, and ac ^ upon the ossicles, 

 io, manubrium or handle, of malleus; 5, long process The former lies in a 

 of incus, terminating in 9, the os orbiculare ; 6, base, groove above the Eusta- 

 ? n _ d8 ,' hea S: of sta P es ' ".Eustachian tube; 12, ex- chian tube, and its 



tendon, passing round a 

 kind of osseous pulley 

 (processus cochleari- 

 the malleus; the stapedius is 

 inner bony wall of the tympanum. Its 

 ------ , ^ck of the stapes near its articulation with 



o S hv m n ^ WaU ' is I )ierced not onlv b Y the oval foramen, 

 by a round opening, the fenestra rotunda, which is closed by 



LmeTmerglven" 111011 *" ^ f SCC ndarv membrana tymjiani I 



14 







