THE SENSE OF HEARING. 647 



incus. Since this process is united at almost a right angle to the 

 stapes bone, the latter is forced toward and into the foramen ovale, 

 thus producing a pressure on the perilymph. With the cessation of 

 the pressure the elastic forces of the membrane and of the ligaments 

 return the handle of the malleus to its former position; by the un- 

 locking of the malleo-incudal joint the entire chain also returns to 

 its former position 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 

 malleus, and as the movement between them takes place around an 

 axis from before backward, it follows that the excursion of the incus 

 and stapes will be less than that of the malleus, while the force will 

 be greater. Hence as the vibrations are transferred from the tym- 

 panic membrane of large area to the base of the stapes of small area 

 (20 to 1.5), they lose in amplitude but increase in force. Their pres- 

 sure on the perilymph is therefore thirty times greater than on the 

 membrana tympani. In addition to its function as a transmitter of 

 vibrations, the chain of bones serves as a point of attachment for 

 muscles which regulate the tension of the tympanic membrane and 

 the pressure on the labyrinth. 



The Function of the Stapedius Muscle. The function of the 

 stapedius muscle is a subject of much discussion. According to 

 Henle, its function is to so adjust the stapes bone that it will be 

 prevented from exerting an undue 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 stapes inward, make 

 it a fixed point, and place the anterior part in such a position that 

 it will vibrate freely and accurately. 



The Function of the Eustachian Tube. In order that the tym- 

 panic membrane may vibrate freely it is essential that the air pressure 

 on both sides shall be equal at all times. This is made possible by 

 the Eustachian tube. Were it not for this passageway, with each 

 inward swing of the membrane the air in the tympanic cavity would 

 be condensed and its pressure raised, in consequence of which the 

 movement of the membrane would be retarded; with each outward 

 swing, the air would be rarified and its pressure lowered below that 

 of the atmosphere, and in consequence the movement outward would 

 be retarded; the maximum response, therefore, of the membrane 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 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 



