194 CONDUCTION THROUGH TYMPANUM. [BOOK in. 



the body ; but it has also a use in transmitting to the tympanic 

 membrane sounds travelling to the ear in certain directions more 

 ivadily than tlio.se coming in other directions. The constriction 

 of the meatus at the junction of the outer and middle third serves 

 as a sort of diaphragm by which waves of sound travelling t<>o 

 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 direc- 

 tions of sounds. 



Sounds however may reach the ear by paths other than the 

 ineatus. 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 influence on the labyrinth itself, but may be 

 also proved by experiment. 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. 



Xot 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 vibra- 

 tions. 



819. 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 stapedius, acts 

 upon the stapes. 



The tensor tympani (Fig. 174) is a slender muscle, lying in 

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



