722 TEXT-BOOK OF PHYSIOLOGY 



the long process of the incus. Since this process is united at almost ' 

 right angle to the stapes bone, the latter is forced toward and into th 

 foramen ovale, thus producing a pressure on the perilymph. With th 

 cessation of the pressure the elastic forces of the membrane and of the lij 

 ments return the handle of the malleus to its former position; by the unlock 

 ing of the malleo-mcudal joint the entire chain also returns to its former posi 

 tion without exerting undue traction on the basal attachment of the stapes 



As the long process of the incus is shorter than the handle of the mallei 

 and as the movement between them takes place around an axis from be for' 

 backward, it follows that the excursion of the incus and stapes will be les 

 than that of the malleus, while the force will be greater. Hence as th 

 vibrations are transferred from the tympanic membrane of large area to th, 

 base of the stapes of small area (*> to 1.5), they lose in amplitude but in 

 crease is force. Their pressure on the perilymph is therefore 13.3 time 

 greater than on the membrana tympani. In addition to its function as i 

 transmitter of vibrations, the chain of bones serves as a point of attachmen 

 for muscles which regulate the tension of the tympanic membrane ant 

 the pressure on the labyrinth. 



The Function of the Stapedius Muscle.-The function of thestapediu. 

 muscle is a subject of much discussion. According to Henle, its functior 

 is so to adjust the stapes bone that it will be prevented from exerting an undu< 

 pressure on the perilymph during the inward excursions of the incus process 

 According to Toynbee, its function is to press the posterior part of the stape< 

 inward, make it a fixed point, and place the anterior part in such a positior 

 that it will vibrate freely and accurately. 



The Function of the Eustachian Tube. In order that the tympanic 

 membrane may vibrate freely it is essential that the air pressure on both 



fl? w -<. eqU f \ at t " tlmCS - This is made P ssible b y the Eustachian 

 tube. Were it not for this passageway, with each inward swing of the mem- 

 brane the air m the tympanic cavity would be condensed and its pressure 

 raised, in consequence of which the movement of the membrane would be 

 :d; with each outward swing, the air would be rarefied and its pressure 

 lowered below that of the atmosphere, and in consequence the movement 

 outward would be retarded; the maximum response, therefore, of the mem- 

 brane to a given vibration could not be attained and the resulting sound 

 would be muffled and indistinct. But as with each vibration of the 

 membrane the air can pass into and out of the tympanum through this 

 partially closed tube, inequalities of pressure are prevented and a free 

 vibration permitted. 



The impairment in the acuteness of hearing which is caused by either 

 a rise or fall of pressure in the middle ear can be shown 



1. By closing the mouth and nose and then forcing air from the lungs 



ressV Eustachian tub e into the tympanum, thus increasing the 



2. By closing the^mouth and nose and then making an effort of deglutition. 



AS is act is attended by an opening of the pharyngeal end of the 

 man tube, the air in the tympanum is partly withdrawn and the 

 pressure lowered. In each instance hearing is impaired. After 

 either experiment the normal condition is restored by swallowing with 

 the nasal passages open. 



