CHAP, iv.] HEARING. 



tympanic membrane sounds travelling to the ear in certain direc- 

 tions more readily than those coming in other directions. The 

 constriction of the meatus at the junction of the outer and mid- 

 dle third serves as a sort of diaphragm by which waves of sound 

 travelling too much out of the line of the meatus are turned 

 back. The external ear, auricle, or pinna has also probably a 

 similar effect, reflecting into the meatus waves which fall upon 

 it in a particular direction or waves of a particular kind. But 

 of these uses, which are of more importance in some animals 

 than in man, we shall speak again in considering the manner in 

 which we recognize the directions of sounds. 



Sounds however may reach the ear by paths other than the 

 meatus. If a tuning-fork be struck and then held near the ear 

 it will after a while cease to be heard, the sound dies away ; but 

 the sound is heard again if the handle of the fork be placed 

 between the teeth ; and when the sound again dies away, it 

 may be revived by gently closing the external meatus, care 

 being taken not to cause compression of the air within. When 

 the tuning-fork is held between the teeth its vibrations are 

 transmitted, through the bones of the head to the tympanic 

 membrane, which thus set in motion acts in the same way as 

 when it is set in motion through the air of the meatus. That 

 the vibrations which thus reach the internal ear are, for the 

 most part at least, conducted through the tympanum, and not 

 brought to bear on the perilymph directly through the bony 

 walls of the labyrinth is not only indicated by the effect just 

 mentioned of closing the meatus, for this could have no influ- 

 ence on the labyrinth itself, but may be also proved by experi- 

 ment. If a style be attached to the stapes laid bare in the skull, 

 the vibrations of a tuning-fork brought into contact with the 

 skull, will lead to corresponding movements of the style. 



Not only may vibrations be transmitted from the skull to 

 the tympanic membrane, but also conversely the vibrations of 

 the membrane, brought about in the usual way through the 

 meatus, may be transmitted to the bones of the skull. If a long 

 tube introduced into one meatus be spoken or sung into, the 

 sounds may be heard by help of a stethoscope placed over 

 various parts of the head. They are heard best perhaps at the 

 opposite meatus ; the vibrations of the bones of the skull set 

 going by one tympanic membrane throw the other tympanic 

 membrane also into vibrations. 



617. Two muscles act upon the auditory apparatus of the 

 tympanum ; one, the tensor tympani^ acts upon the malleus and 

 hence upon the tympanic membrane, the other, the stapediu*, 

 acts upon the stapes. 



The tensor tympani (Fig. 176) is a slender muscle, lying in 

 a groove above the bony canal of the Eustachian tube, and having 

 very much the direction of that tube. The tendon in which it 



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