RE G ULA TION OF PRESS URE IN THE TYMPANUM. 1 1 6 1 



Secchi's views have, up to a certain point, received confirmation from 

 the recent careful observations of Bezold. 1 The latter has found that, 

 in disease causing rigidity or loss of the ossicles, the hearing is seriously 

 affected as regards the lower and middle notes, while the upper notes 

 are heard perfectly well. Thus the sufferers may not hear loud thunder, 

 but the whistling of a canary may be clearly heard at a considerable 

 distance. Hence Bezold holds that the ossicles are indispensable for 

 the transmission of the lower notes, but take no part in the conduction 

 of the upper ones. 



There is still another route by which sound pressures may be trans- 

 mitted from the external air to the labyrinth, namely, by the bones of 

 the head. This is the case not only when the sounding body is applied 

 to the cranium, but also when the sounds are transmitted through the 

 air. Politzer 2 found that when the external auditory meatus on both 

 sides was filled with glycerine, and the nose and mouth firmly closed, 

 conversation could be understood at a distance of - 5 metre. It is also 

 known that tones are perceptible even in cases where the stapes is 

 ankylosed to the edges of the fenestra ovalis, if the tones are of suffi- 

 cient intensity. As sound-waves are conducted through the solid and 

 fluid parts of the body, they must directly reach the labyrinth without 

 passing through the external or middle ears. Therefore, if the structures 

 in the labyrinth are normal, it is impossible to exclude completely 

 sound-waves from being perceived when transmitted by the bones of 

 the head. 



Regulation of atmospheric pressure in the tympanum. — As the 

 membrana tympani is very thin and fibrous, the pressure of the air on 

 each side of it must affect its capability of vibrating when subjected to 

 periodic sound pressures. On one side it is freely exposed to the air, 

 while on the other it is in contact with a confined mass of air in the 

 tympanic cavity. As the air in the latter might be to some extent 

 absorbed, and consequently have its pressure reduced, and as, on the 

 other hand, the pressure of the air on the outer surface is liable to 

 considerable variations, there must be some arrangement for establishing 

 from time to time an equilibrium of pressure on the two sides of the 

 membrane. This is accomplished by the Eustachian tube, passing from 

 the tympanic cavity to the posterior part of the pharynx. This tube 

 occasionally opens so as to allow the air of the pharynx, which communi- 

 cates by the oral and nasal openings, to pass into the tympanum, and 

 to exert its pressure on the inner surface of the membrana tympani. It 

 is usually held that the Eustachian tube is closed during ordinary breath- 

 ing, and that it is opened only at the moment of deglutition. Cleland 3 

 and others, 4 however, maintain that the tube is open during rest, and 

 closed during deglutition. 



An experiment performed by Politzer 5 throws some light upon the matter 

 now under consideration. He constructed a small glass manometer, the short 

 limb of which passed through an indiarubber cork. A few drops of a 

 coloured fluid were introduced into the manometer. The cork was pressed 



1 "Ueber d. functionelle Pruf. d. menschl. Gehor.," Wiesbaden, 1897, S. 118-122. 



2 Arch./. Ohrenh., 1864, Bd. i. S. 350. 



3 Jourti. Anat. and Physiol., London, vol. iii. p. 97. 



4 Luschka holds that the tensor opens the tube and the levator closes it. " Der 

 Schlundkopf d. Menschen," Tubingen, 1868, S. 45 and 47. See also Hammerschlag, Wien. 

 med. Wchnschr., Nos. 39 and ■lO, 1896; and Lucae, Arch./. Ohrenh., Bd. xlii. S. 178. 



5 " Diseases of the Ear," p. 64. 



