EXTERNAL AND MIDDLE PORTIONS OF THE EAR 769 



resistance which finds its origin in several conditions. Among these 

 we have cited the peculiarities in the structure and position of the ear- 

 drum and also the unusual characteristics of the lever formed by the 

 ossicles. In addition, Helmholtz has called attention to the fact that 

 the articulation between the malleus and incus may be broken at any 

 time by unusually strong inward movements of the eardrum. 1 The 

 head of the malleus is then forced outward so far that the incus cannot 

 follow it. Doubtlessly , this dislocation serves to protect the internal ear 

 against sounds of extraordinary striking force. As we shall see later, 

 an additional factor of safety has been provided in the shape of the 

 stapedius muscle, the contraction of which pulls the head of the stapes 

 over so that this bone presses more firmly upon the membrane closing 

 the fenestra ovalis, thereby diminishing its vibratory qualities. 



The Eustachian Tube. A membrane, such as the eardrum, is 

 capable of developing the most perfect vibrations only when the pres- 

 sure upon its two surfaces is equal. If the tympanic cavity were 

 absolutely closed, the air contained therein would be absorbed in the 

 course of time, establishing a rarefaction which in turn would give rise 

 to an inward bulging of the eardrum, and a diminution in its oscillatory 

 power. Under ordinary conditions, however, a result of this kind 

 is obviated by the fact that the tympanum is connected with the 

 pharyngeal cavity by means of a membranous communication, known 

 as the Eustachian tube. While the pharyngeal end of this channel 

 is kept closed under ordinary conditions, it may be opened at any time 

 by the act of swallowing which involves a contraction of the muse, tensor 

 veli palatini. This permits of an interchange of air in both directions. 

 The closure of this orifice results immediately upon the cessation 

 of this muscular effort on account of the elastic recoil of its valve-like 

 lips, situated inside the ostium. 



If we enter a tunnel in which the pressure is above that of the 

 atmospheric air, the tympanic membrane is forced inward. This gives 

 rise to a peculiar local sensation of pressure as well as to a diminution 

 in the acuity of hearing. The tube is then opened by the act of 

 swallowing which allows the required amount of air to rush into the 

 tympanum. In a similar way, a diminution in the atmospheric pres- 

 sure gives rise to an outward displacement of the eardrum which is 

 remedied immediately by permitting air to escape from the tympanum. 

 A condition of the first kind may be set up very easily by swallowing 

 during the act of inspiration while the lips and nostrils are held shut. 

 The opposite condition may be produced by swallowing during expira- 

 tion while the lips and nostrils are kept closed. 



Although this tube serves chiefly as a means for the ventilation 

 of the tympanum, it also forms a natural outlet for excess secretions. 

 Both functions are greatly impaired during catarrhal affections of the 



1 This hypothesis has been criticized by von Frey (Pfliiger's Archiv, cxxxix, 1911, 

 548) upon the ground that the malleus and incus are not united by a true joint, 

 but are more or less ankylosed. 



49 



