CHAP, iv.] HEARING. 995 



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. 171 St.) and being inserted into 

 the head of the stapes (Fig. 177 ST). It is supposed to regu- 

 late the movements of the stapes, and especially to prevent the 

 foot-plate being driven too far into the fenestra ovalis during 



ST 



EIG. 177. THE STAPES IN POSITION. Much magnified. (Schwalbe.) 



1. The end of the shaft of the incus. 2. Its expansion or os orbiculare. 

 2'. The articular cartilage of the same. 3. The capitulum of the stapes. 3'. Its 

 articular cartilage. 4. The hoops of the stapes. 5. The foot-plate of the stapes. 

 5'. Its articular cartilage. 6. The membrane of the fenestra ovalis. 



ST. The tendon of the stapedius muscle attached to the capitulum of the 



stapes. 



large or sudden movements of the tympanic membrane. Con- 

 tractions 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 mem- 

 branous 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. 



618. 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 to- 

 wards the median line (Fig. 171, E.t.*) to open at the side 

 of the upper part of the pharynx. The mucous membrane 



