8o6 



CONDUCTION OF SOUND IN THE LABYRINTH. 



increased air space, obtained by its communication with the mastoid cells, permits 

 free vibration of the membrana tympani. The six sides of the tympanum have 

 important practical relations. It is about half an inch in height, and one or two 

 lines in breadth, i.e., from without inwards. Its roof is separated from the cavity 

 of the brain by a very thin piece of bone, which is sometimes defective, so that 

 encephalitis may follow an abscess of the middle ear. The outer wall is formed by 

 the membrana tympani, while on the inner wall are the fenestra ovalis and rotunda, 

 the ridge of the aqueductus Fallopii, the promontory, and the pyramid. The floor 

 consists of a thin plate of bone, which roofs in the jugular fossa and separates it 

 from the jugular vein. Fractures of the base of the skull may rupture the carotid 

 artery or internal jugular vein ; hence, haemorrhage from the ears is a bad symptom 

 in these cases. Caries of the ear may extend to other organs. The anterior wall 

 is in close relation with the carotid artery, while the posterior communicates with 

 the mastoid cells, so that fluids from the middle ear sometimes escape through the 

 mastoid cells.] 



That the air in the tympanum can communicate its vibrations to the membrane of the fenestra 

 rotunda is tme (p. 797, 3), but normally this is so slight, when compared with the conduction 

 through the auditory ossicles, that it scarcely need be taken into account. 



Structure. The tube and tympanum are lined by a common mucous membrane, covered by 

 ciliated epithelium, while the membrana is lined by a layer of squamous epithelium. Mucous 

 ^^ glands were found by Trbltsch and Wendt in the mucous membrane. 



^^ [The epithelium covering the ossicles and tensor tympani is not ciliated.] 



^k Pathological. The tube is often occluded, owing to chronic catarrh 



and narrowing from cicatrices, hypertrophy of the mucous membrane, 

 or the presence of tumours. The deafness thereby produced may often 

 be cured by cathetcrising the tube from the nose (fig. 588). Effusions 

 into or suppuration within the tympanum of course paralyse the sound- 

 conducting mechanism, while inflammation often causes subsequent 

 affections of the plexus tympanicus. If the temporal bone be destroyed 



<2 



Rrohne & Sesett^ " 



Fig. 588. 

 Eustachian catheter, 



Fig. 589. 

 Politzer's ear bag. 



by progressive caries within the tympanum, inflammation of the neighbouring cerebral struc- 

 tures may occur and cause death. 



[Methods. Not unfrequently the aurist is called upon to dilate the Eustachian tube, which 

 in certain cases requires the use of a Eustachian catheter introduced into the tube along the 

 floor of the nose (ng. 588). At other times he requires to fill the tympanic cavity with air, 

 which is easily done by means of a Politzer's bag (hg. 589). The nozzle is introduced into one 

 nostril, while the other nostril is closed, and the patient is directed to swallow, while at the 

 same moment the surgeon compresses the bag, and the patient's mouth being closed, air is 

 forced through the open Eustachian tube into the middle ear. Sometimes a small curved 

 narrow manometer, containing a drop of coloured water, is placed in the outer ear (Politzer). 

 Normally, when the patient swallows, the fluid ought to move in the tube.] 



412. CONDUCTION OF SOUND IN THE LABYRINTH. The vibrations 

 of the foot of the stapes in the fenestra ovalis give rise to waves in the perilymph 

 within the inner ear or labyrinth. These waves are so-called "flexion waves," i.e., 

 the perilymph moves in mass before the impulse of the base of the stapes. This is 

 only possible from the existence of a yielding membrane that filling the fenestra 

 rotunda, and sometimes called the membrana secundaria, which during rest bulges 



