1334 THE EUSTACHIAN TUBE. [BOOK in. 



nerve from the otic ganglion, and its activity is regulated by reflex 

 action, vibrations of the tympanic membrane starting the afferent 

 impulses. In some persons the muscle seems to be partly under 

 the dominion of the will, since a peculiar crackling noise which 

 these persons can produce at pleasure appears to be caused by 

 contraction of the tensor tympani. 



The stapedius is a small muscle imbedded in the bone of the 

 median wall of the tympanum, the tendon issuing by a hole close 

 to the fenestra ovalis (Fig. 166 St.) and being inserted into the 

 head of the stapes (Fig. 172 ST). ' It is supposed to regulate the 

 movements of the stapes, and especially to prevent the foot plate 

 being driven too far into the fenestra ovalis during large or 

 sudden movements of the tympanic membrane. Contractions of 

 the muscle pull the front part of the stapedial foot plate towards 

 the tympanum, the hind part being thereby pressed somewhat 

 into the labyrinth and the whole membranous ring round the foot 

 being rendered more tense ; but the total effect is to diminish the 

 pressure in the labyrinth. It perhaps may be regarded as the 

 antagonist of the tensor tympani. It is governed by fibres from 

 the facial nerve. 



820. The cavity of the tympanum is, as we have seen, 

 continuous with the Eustachian tube. The walls of the tube in 

 the first third of its length adjoining the tympanum are osseous, 

 but in the remaining two-thirds are cartilaginous and mem- 

 branous. The tube, whose lumen is of varying diameter and 

 special shape, passes obliquely forwards, downwards, and towards 

 the median line (Figs. 166, 167 E.t.} to open at the side of the upper 

 part of the pharynx. The mucous membrane lining the tube 

 consists of a ciliated epithelium resting on a dermis rich in re- 

 ticular 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 sometimes happens 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 



