996 THE EUSTACHIAN TUBE. [BOOK in. 



lining the tube consists of a ciliated epithelium resting on a 

 dermis rich in reticular and adenoid tissue, and bearing glands. 

 The action of the cilia is such that the movement which they 

 effect is directed from the tympanum to the pharynx. The 

 mucous membrane lining the tympanum is a continuation of 

 that lining the tube and, like that, ciliated except over the 

 tympanic membrane, the chain of ossicles, and probably some 

 other parts ; in these situations the epithelium consists of a 

 single layer of flat non-ciliated cells, and a similar epithelium 

 lines the antrum and mastoid cells which continue the cavity of 

 the tympanum backwards and upwards. 



One use of the Eustachian tube is to carry down to the 

 pharynx the fluid, normally very small in amount, which is 

 secreted by the mucous lining of the tympanum, but a far more 

 important use is that of placing the air in the tympanum in 

 communication with that in the pharynx and so with the 

 external air, by which means the pressure on the two sides of 

 the tympanic membrane is equalized. If as happens sometimes 

 the tube is definitely closed, the absorption of the gases in the 

 air at first present in the tympanum diminishes the pressure on 

 the inner side of the tympanic membrane, and so interferes with 

 the vibrations of the 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 nor- 

 mally closed or open. It is undoubtedly opened during the act 

 of swallowing, and during the act, by the action of certain mus- 

 cles of the 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 prac- 

 cally closed ; sounds for instance generated in the pharynx 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 effected by 

 swallowing, is sufficient to keep the pressure within the tym- 

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

 external atmosphere is rapidly increased or diminished, tempo- 



