THE SENSES 



955 



when a swallowing 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 compressed the excess of pressure is on the 

 external surface of the membrane. The feeling of uncomfortable 

 tension is relieved in both cases by swallowing 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 rarefac- 

 tion of the air in the 

 tympanum. The 

 patient instinctively 

 makes efforts which 

 increase the pharyn- 

 geal pressure from 

 time to time so as to 

 open the tube. 



The loosely-j ointed 

 chain of ossicles is 

 steadied and its 

 movements directed 

 by ligaments and by 

 the tension of its ter- 

 minal membranes. It 

 forms a kind of bent 

 lever, by which the 

 oscillations of th.e 

 membrana tympani 

 are transferred to the 

 membrane covering 

 the oval foramen, and 

 at the same time re- 

 duced in size. Two 

 slender muscles, the 

 tensor tympani and 

 stapedius, contained 

 in the tympanic 

 cavity, are also con- 

 nected with and may 

 act upon the ossicles. 

 The former lies in a 



FIG. 420. 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, long process of incus, terminating 

 in 9, the os orbiculare ; 6, base, and 8, head, of stapes ; 

 n, Eustachian tube ; 12, external auditory meatus ; 

 13, membrana tympani ; 3, upper, and 14, lower, part of 

 tympanum. 



groove above the 

 Eustachian 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 \vall 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 



