896 SOUND-CONDUCTION IN THE LABYRINTH. 



hearing this slight conduction is of but little significance in comparison with that 

 of the ossicles. 



The Eustachian tube and the tympanum have a common mucous membrane ; 

 the parts within the tympanum are lined by the mucosa. The epithelium is 

 composed of ciliated columnar cells ; it is not ciliated on the surface of the ossicles 

 and the promontory. Troltsch and Wendt found racemose mucous glands in 

 the mucous membrane. 



Pathological. Among the diseases of the Eustachian tube, obstruction 

 attending chronic catarrhal conditions, and narrowing from scars, hypertrophy 

 of the mucous membrane or pressure by tumors may be mentioned. The im- 

 pairment of hearing thus produced can often be corrected by catheterizing the 

 tube through the nares. Effusions and collections of pus in the tympanum must, 

 of course, disturb the normal function of all the sound-conducting parts within 

 the tympanum. Inflammatory processes often have also injurious effects upon 

 the tympanic plexus. Moreover, progressive destruction of the temporal bone 

 by caries, commencing in the tympanic cavity may finally cause fatal inflam- 

 mation of the neighboring portions of the brain. 



SOUND-CONDUCTION IN THE LABYRINTH. 



The oscillations of the basal plate of the stapes in the fenestra ovalis 

 of the vestibule produce waves in the fluid of the labyrinth, so-called 

 flexion- waves, that is the labyrinthine fluid moves as a whole before each 

 impulse of the stapes. This yielding of the fluid is 

 possible only because in one place a flexible mem- 

 brane, the membrane of the fenestra rotunda (of the 

 cochlea) or secondary tympanum, which, when at 

 rest, projects into the scala tympani, can be forced 

 outward toward the tympanic cavity by the move- 

 ment of the stapes (Fig. 314, r). These waves must 

 correspond in number and intensity to the move- 

 Lab- ments of the auditory ossicles, and must also excite 

 dow th ieadin e True/the the terminat i ns of the auditory nerve, which float 

 vestibule, ' the 'codiiea! free in the fluid of the labyrinth. 

 ri 8 <JFS5 horiz'cSal As both the cochlea anteriorly and the semicircular 



(J) semicircular canal canals posteriorly communicate with the saccules of 

 the vestibule, the fluid of which first receives the im- 

 pulse of the vibrations, the movement of the fluid must 

 be propagated through these canals. In the cochlea the movement passes 

 upward from the sacculus (hemisphaericus) through the scala vestibuli 

 to the apex of the cochlea, here through the helicotrema into the scala 

 tympani, at the extremity of which the membrane of the fenestra rotunda 

 moves outward. In a similiar manner the wave-motion commencing 

 in the utricle (sacculus hemiellipticus) passes along through the semi- 

 circular canals. Thus, Politzer saw the fluid of the labyrinth mount 

 upward into the superior canal (which was exposed) when he caused 

 contraction of the tensor tympani by stimulating the trigeminus, 

 which must force the base of the stapes against the labyrinthine fluid, 

 with each sound-vibration of the tympanic membrane. 



Hensen showed that a membrane that is set in motion by water exercises 

 a strong attraction. This attraction may be noticed also on the oval membrane 

 of the labyrinth, which is weighted by the base of the stapes; the fluid must, 

 consequently, move toward the stapes, and then away from it. Otoliths are 

 probably also under the influence of this attraction, and their mechanical action 

 upon the endings of the auditory nerve is thus clearly explained. 



