CONDUCTION THROUGH THE TYMPANUM. 603 



the incus, being fixed by a firm joint to the head of the malleus, 

 must follow these movements, and thus they cause the little oval 

 foot-piece of the stirrup to press in or to draw out the membrane 

 which separates the tympanum from the vestibule. Thus the 

 vibrations of the air communicated to the tympanic membrane 

 are conveyed across the cavity of the drum to the liquid in the 

 labyrinth. 



A small muscle the stapedius is attached to the stapes near 

 its junction with the incus, and pulls upon it in such a direction 

 that the bone is drawn out of the line of motion. This action, 

 possibly, has the effect of reducing the effect of the more ample 

 vibrations of the tympanic membrane, which might have too 

 great an effect upon the liquid in the labyrinth. 



The tympanum is connected with the pharynx by means of the 

 Eustachian tube, which, though habitually closed, is opened for 

 a moment by swallowing and other motions of the pharynx. On 

 these occasions air can pass in or out of the tympanum easily, so 

 that the pressure on the two sides of the membrane of the drum 

 is equalized. When there is too much or too little air in the 

 tympanic cavity, the tympanic movements are impeded. This 

 difficulty is felt when one has a bad cold ; the tube is occluded 

 by the inflammatory swelling of the mucous membrane. Or 

 when one performs what is known as Valsalva's experiment, i. e., 

 to hold the nose and violently puff air into it ; when the tubes 

 are blown open, too much air is often retained in the tympanum, 

 so that the pressure from within is higher than that from with- 

 out, and hearing becomes dull. If, now, the act of swallowing 

 be performed, the feeling of tension leaves the ears and hearing 

 becomes as acute as before. 



The Eustachian tube also acts as a way of escape for any fluid 

 that may be secreted by the epithelial lining of the tympanic 

 cavity. This fluid is so minute that the periodic opening of the 

 tube suffices, under ordinary circumstances for its complete 

 escape. When increased by disease, however, it may collect in 

 the tympanum and require catheter ization. 



If the tube were permanently open, we should suffer from two 

 great disadvantages. In the first place, at every breath, even 



