746 HEARING [CH. LIV. 



middle ears are conducting; the internal ear is conducting and 

 receptive. In the external ear the vibrations travel through air ; in 

 the middle ear through solid structures membranes and bones ; and 

 in the internal ear through fluid, first through the perilymph on the 

 far side of the fenestra ovalis ; and then the vibrations pass through 

 the basilar membrane, and membrane of Eeissner, and set the endo- 

 lymph of the canal of the cochlea in motion. 



This is the normal way in which the vibrations pass, but the endolymph may be 

 affected in other ways, for instance through the other bones of the head ; one can, 

 for example, hear the ticking of one's watch when it is placed between the teeth, 

 even when the ears are stopped. From this fact is derived a valuable practical 

 method of distinguishing in a deaf person what part of the organ of hearing is at 

 fault. The patient may not be able to hear a watch or a tuning-fork when it is held 

 close to the ear ; but if he can hear it when it is placed between his teeth, or on his 

 forehead, the malady is localised in either the external or middle ear ; if he can hear 

 it in neither situation, it is a much moie serious case, for then the internal ear or the 

 nervous mechanism of hearing is at fault. In disease of the middle ear the hearing 

 of low tones is especially affected ; high tones appear to be transmissable by bone- 

 conduction more readily than low. 



In connection with the external ear there is not much more to be 

 said ; the pinna in many animals is large and acts as a kind of natural 

 ear-trumpet to collect the vibrations of the air ; in man this function 

 is to a very great extent lost, and though there are muscles present to 

 move it into appropriate postures, they are not under the control of the 

 will in the majority of people, and are functionless, ancestral vestiges. 



In the middle ear, however, there are several points to be con- 

 sidered, namely, the action of the membrana tympani, of the ossicles, 

 of the tympanic muscles, and of the Eustachian tube. 



The Membrana Tympani. This membrane, unlike that of 

 ordinary drums, can take up and vibrate in response to, not only its 

 own fundamental tone, but to an immense range of tones differing 

 from each other by many octaves. This would clearly be impos- 

 sible if it were an evenly stretched membrane. It is not evenly nor 

 very tightly stretched, but owing to its attachment to the chain of 

 ossicles it is slightly funnel-shaped : the ossicles also damp the con- 

 tinuance of the vibrations. 



When the membrane gets too tightly stretched, by increase or 

 decrease of the pressure of the air in the tympanum, then the sense 

 of hearing is dulled. The pressure in the tympanic cavity is kept 

 the same as that of the atmosphere by the Eustachian tube, which 

 leads from the cavity to the pharynx, and so to the external air. 

 The Eustachian tube is not, however, always open ; it is opened by 

 the action of the tensor palati during swallowing. Suppose it were 

 closed owing to swelling of its mucous membrane this often 

 happens in inflammation of the throat the result would be what is 

 called Eustachian or throat deafness, and this is relieved by passing 

 a catheter so as to open the tube. When the tube is closed, an 



