THE SENSES 843 



more powerful movements of the stapes. Even the so-called 

 cranial conduction of sound when a tuning-fork is held between 

 the teeth or put in contact with the head, which was at one 

 time supposed to be due solely to direct transmission of the 

 vibrations through the bones of the skull to the liquids of 

 the labyrinth or the end-organs of the auditory nerve, has 

 been shown to take place, in great part, through the mem- 

 brana tympani and ossicles ; the vibrations travel through the 

 bones to the tympanic membrane, and set it oscillating. So 

 that this test, when applied to distinguish deafness caused by 

 disease of the middle ear from deafness due to disease of the 

 labyrinth or the central nervous system may easily mislead, 

 although it enables us to say whether the auditory meatus is 

 blocked (by wax, e.g.) beyond the tympanic membrane. 



When a tuning-fork is held between the teeth, a part of the sound 

 passes out of the ear from the vibrating membrana tympani ; if one 

 ear is closed, the sound is heard better in this than in the open ear. 

 If the tuning-fork is held between the teeth till, with both ears open, 

 it becomes inaudible, it will be heard for a short time if one or both 

 ears be stopped ; and when in this position the sound again becomes 

 inappreciable, it can still be caught if the handle be introduced into 

 the auditory meatus. 



A membrane like a drum-head has a note of its own, which it gives 

 out when struck, and it vibrates more readily to this note than to any 

 other. It would evidently be a serious disadvantage if the tympanic 

 membrane, whose office it is to receive all kinds of vibrations, and 

 respond to all, had a marked fundamental tone which would be con- 

 tinually obtruding itself among other notes. The difficulty is obviated 

 by the damping action of the ossicles and the liquids of the laby- 

 rinth on the movements of the membrane, which in addition is not 

 stretched, but lies slackly in its bony frame, so that when the handle 

 of the malleus is detached from it, it retains its shape and position. 



The tensor tympani, when it contracts, pulls inwards the handle 

 of the malleus, and thus increases the tension of the tympanic mem- 

 brane. The precise object of this is obscure. It has been suggested 

 that damping of the movements of the auditory ossicles is thus 

 secured. Another theory is that the increased tension of the mem- 

 brane renders it more capable of responding to higher tones, and 

 that the muscle thus acts as a kind of accommodating mechanism. 

 But Hensen has observed that the tensor only contracts at the begin- 

 ning of a sound, and relaxes again when the sound is continued ; 

 and this is difficult to reconcile with either of these hypotheses. 

 The muscle is normally excited reflexly through the vibrations of the 

 membrana tympani, but some individuals have the power of throwing 

 it into voluntary contraction, which is accompanied by a feeling of 

 pressure in the ear and a harsh sound. The function of the stapedius 



