USE OF THE MEMBRANA TYMPANI. 405 



boundary, or membrana tympani. Lastly, the fenestra rotunda, the 

 supposed path of the vibrations propagated through the air in the tym- 

 panum, is, in comparison with the fenestra ovalis, small, retired in 

 position, and has but a limited connection with the cavities of the laby- 

 rinth; in many animals endowed with a highly acute auditory sense, it 

 is even placed in a neighboring cavity, which communicates with the 

 tympanum through a very narrow fissure. It will thus be seen that, 

 whilst the tympanic apparatus presents a combination of membranous 

 and solid materials, well adapted for the conduction of sonorous vibra- 

 tions, the tympanic cavity seems to possess contrivances specially cal- 

 culated to impair the conducting properties of its contained air. 



The membrana tympani is, moreover, a defensive structure, prevent- 

 ing the entry of foreign bodies into the tympanum ; by it also the 

 interior of this cavity is kept in a state of moisture. The interposi- 

 tion of the tympanic cavity between the outer and inner parts of the 

 ear, serves, by the maintenance of warm air, at a uniform temperature, 

 in that situation, to preserve the delicate structures of the labyrinth 

 from exposure and changes of temperature, conditions essential for the 

 performance of their functions. 



By means of the Eustachian tube, which leads into the upper part 

 of the pharynx, equal atmospheric pressure on the two sides of the 

 membrana tympani, is maintained ; the freedom of movement of the 

 membrane, both in vibrating and in changing its degree of tension, is 

 secured, and its undue tension, which would arise if either of its sides 

 were subjected to excessive or deficient pressure, is prevented. More- 

 over, the existence of this outlet, protects the membrana tympani 

 against the effects of external atmospheric shocks, and prevents acci- 

 dental rupture or laceration. The Eustachian tube also serves as a 

 conduit for the escape of the fluids secreted in the tympanum and 

 mastoid cells ; this is aided by the cilia covering its interior. The 

 pharyngeal ends of those tubes are only open in swallowing and 

 yawning; they can, moreover, be opened at will, by closing the mouth 

 and nose, and then forcing air into them, or by performing the act of 

 deglutition, when a clicking noise is heard, from the motion of the tym- 

 panic membrane and ossicles. The Eustachian tube is, however, always 

 pervious in the healthy condition, and this is essential for the due per- 

 formance of its functions ; for when it is obstructed by accumulation 

 of the fluids secreted by the parts, by the pressure of enlarged tonsils, 

 by swelling of the mucous membrane at its orifice, or by any other 

 cause, deafness, in proportion to the amount of obstruction, is pro- 

 duced. It was formerly supposed, that these tubes conduct one's own 

 voice to the ears ; this notion is, however, erroneous ; the ticking of 

 a watch introduced into the mouth, is but feebly heard ; moreover, 

 obstruction of the tubes does not render the hearing of one's own voice 

 more difficult than that of other sounds. 



The state of tension of the membrana tympani is regulated by the 

 action of the muscles connected with the malleus and stapes. There 

 can be no doubt that when the tensor tympani contracts, this mem- 

 brane is drawn inwards, and that its tension is thereby increased. The 

 action of the stapedius on the membrana tympani is doubtful ; some 



