CHAP, iv.] HEARING. 197 



membrane. Moreover it is desirable that general changes of 

 pressure in the external atmosphere should be rapidly followed 

 by corresponding changes in the pressure within the tympanum, 

 since the tympanic membrane would not vibrate normally if any 

 marked difference of pressure on the two sides were brought 

 about ; and this would result if the way from the tympanum to 

 the external air through the tube were blocked. 



The lumen of the tube has in its lower part the form of an 

 obliquely vertical slit, the sides touching or nearly so ; and much 

 dispute has taken place as to whether the tube is normally closed 

 or open. It is undoubtedly opened during the act of swallowing, 

 and during the act, by the action of certain muscles of th e palate, 

 air is forced up into the tympanum. It may be opened also by a 

 forced inspiration or a forced expiration when the nose and mouth 

 are kept closed ; in the former case the pressure of the air in the 

 tympanum is diminished, in the latter case increased. Although 

 under normal circumstances the lumen is so far patent as to allow 

 the escape of the fluid driven by the cilia, the evidence goes to 

 shew that it is practically closed ; sounds for instance generated in 

 the pharnyx do not throw the tympanic membrane into vibrations 

 in such a way as they would do if the tube were thoroughly open. 

 Apparently the occasional opening, such as that elected by 

 swallowing, is sufficient to keep the pressure within the tympanum 

 at its proper level. When the general pressure of the external 

 atmosphere is rapidly increased or diminished, temporary deafness, 

 especially to low notes, frequently ensues, in consequence of the 

 pressure within the tympanum not following the changes of the 

 pressure without. This however is soon remedied by the act of 

 swallowing, which opens the tube and thus equalises the pressure. 



An abnormal permanency in the closure of the tube is recog- 

 nized as a cause of deafness, and may be remedied by catheterism 

 of the* tube, that is to say, opening up the tube by passing an 

 instrument into it from the pharnyx. 



