io66 



THE SENSES 



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

 ear approximately that oj the atmosphere. ~in a pauooiT 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- 

 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 

 cavity, are also con- 

 nected with and may 

 act upon the ossicles. 

 The former lies in a 

 groove above the Eusta- 

 chian tube, and its 

 tendon, passing round a 

 kind of osseous pulley 

 (processus cochleari- 



formis), is inserted into the handle of the malleus; the stapedius is 

 lodged in a hollow of the inner bony wall of the tympanum. Its 

 tendon is attached to the neck of the stapes near its articulation with 

 the incus. This inner wall is pierced not only by the oval foramen, 

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

 a membrane to which the name of secondary membrana tympani is 

 sometimes given. 



The internal ear consists of the bony labyrinth, a series of curiously 

 excavated and communicating spaces in the substance of the petrous 

 portion of the temporal bone, filled with a liquid called the perilymph, 

 in which, anchored by strands of connective tissue, floats a correspond- 

 ing series of membranous canals (the membranous labyrinth), filled 

 with a liquid called endolymph. The labyrinth of the internal ear is 



Fig- 454- Tympanum of Left Ear, showing the 

 Ossicles (Morris), i, superior, and 4, external, 

 ligament of malleus; 2, head; 7, short process, and 

 10, manubrium or handle, of malleus; 5, longprocess 

 of incus, terminating in 9, the os orbiculare; 6, base, 

 and 8, head, of stapes; n, Eustachian tube; 12, ex- 

 ternal auditory meatus; 13, membrana tympani; 

 3, upper, and 14, lower, part of tympanum. 



