EUSTACHIAX TUBE. TYMPANIC CAVITY. 895 



tympanic membrane can be transmitted with less impairment to the 

 ossicles than if the tube were open and the air allowed to escape through 

 it during the vibrations. If, however, the tympanum were permanently 

 closed, the air within it would soon be so rarefied, that the tympanic 

 membrane would be drawn inward, under abnormal tension, and hard- 

 ness of hearing would result. The tube serves, moreover, as a drainage- 

 canal for the secretion of the tympanic cavity by means of the ciliated 

 epithelium. 



If, after destruction of the tympanic cavity, gas is allowed to stream into 

 the ear of a narcotized dog, through the external canal, it passes through the 

 tube into the throat only when the tensor tympani contracts. 



The tube opens its valvelike mechanism more easily in the direction of the 

 pharynx than in the opposite direction. The valve is placed behind the orifice 

 of the tube ; after each opening of the mouth of the tube the valve is again closed 

 through the elasticity of the tube-walls. 



A tuning-fork held before the nostrils is heard more strongly during a swallow- 

 ing movement, because the tube is opened. One's own voice seems deafening 

 at the moment that the tube is opened by an influx of air, and the voice seems 

 to sound as if within the ear. Patulousness of the tube as a result of pathological 

 conditions may produce a similar result autophony. The 

 pulsation of the vessels and the respiratory sounds are then 

 also abnormally distinct. 



If the act of swallowing is performed slowly in the 

 pharynx, while the tensors of the palate are stretched, a 

 sharp hissing, or loud crackling noise, is heard distinctly. 

 This sounds much like the noise produced by forcing 

 saliva between the incisor teeth by pushing the tongue for- 

 ward when the mouth is closed, and it results from the 

 separation of the moistened walls of the tube from each 

 other. Another person can hear this noise by applying his 

 ear, or by using a stethoscope. It was formerly thought to 

 be a cracking of the joints of the ossicles through the 

 action of the tensor tympani. 



In Valsalva's experiment air enters the tube as soon as FIG. 323. Section 

 the air-pressure equals betw r een TO and 40 mm. of mercury. ^ r u ug )r>- Eustac *\ la . n 



TT j i_ j/i. Tj-1 j .c .LI lube (Diagrammatic): 



Under such conditions Landois heard first the same noise, m , median plate; /, 

 and then he felt suddenly the increased tension of the lateral plate; *, mar- 

 tympanic membrane due to the entrance of air into the SoR C "/ t SSiocf flS 

 tympanic cavity. During forced inspiration, while the pa i a te; L, lumen 

 mouth and nostrils are held closed, air is sucked out, and 

 finally the tympanic membrane is drawn inward. 



The elevator of the veil of the palate forms in this situation the levator- 

 cushion as it passes under the floor of the pharyngeal orifice of the tube (Fig. 330). 

 Consequently, when this muscle contracts, and its belly thickens (in the com- 

 mencement of the act of swallowing), and also with every elevation of the soft 

 palate during inspiration, the lower wall of the pharyngeal opening is forced 

 upward, and the opening is narrowed. The subsequent contraction of the tensor 

 of the veil of the palate, in the further course of the act of swallowing, then dilates 

 the tube. (The subject is further discussed with the act of swallowing.) The 

 result is that by this action of the levator the tension of the air in the tympanum 

 is at first increased; it is then diminished by the action of the tensor, as may 

 be recognized under favorable conditions from corresponding movements of the 

 tympanic membrane. 



The tympanic cavity forms a protective chamber for the auditory 

 ossicles and their muscles. Its air-capacity, amplified by the communi- 

 cations with the mastoid cells, permits free oscillations of the tympanic 

 membrane. 



The assumption that the tympanum strengthens by resonance the sound- 

 vibrations that strike the ear, for the purpose of delicate hearing, must be con- 

 sidered erroneous. That, further, the air of the tympanum can transmit vibra- 

 tions to the membrane of the iVm-stru rotunda must be admitted, but with normal 



