232 PHYSIOLOGY. 



which the orbicular and stirrup bones are attached, moves through a 

 greater space than that which receives the impulse of the head of the 

 malleus. Thus a very small degree of motion, communicated by the 

 head of the malleus to the body of the incus, is greatly increased in 

 the extremity of the long process of the incus ; and consequently this 

 mechanism assists greatly in giving strength to the vibrations trans- 

 mitted to the internal ear. 



12^ Now in distinguishing low sounds, the drum head is 

 supposed to be put upon the stretch, chiefly by the aid of one 

 little muscle, called tensor tympani, which is attached to the 

 malleus, and the centre of the membrane, so that when it 

 acts, it pulls the long handle of that bone, and draws the tym- 

 panum inwards. Thus being made tense, it is prepared for 

 the reception of low sounds. When sounds are so acute as 

 to be painful, and we wish to diminish their intensity, this 

 muscle relaxes, and the tympanum becoming comparatively 

 flaccid, a deadened muffled impression is transmitted to the 

 brain, precisely as when we muffle a drum. This relaxation 

 is also aided by a muscle expressly prepared for that pur- 

 pose, called laxator tympani, which arises from the temporal 

 bone and is inserted into the handle of the malleus, so as to 

 pull it forwards^ 



13. \ We traced the progress of sound through the external 

 ear, the auditory passage, and the chain of bones, to the 

 stapes. Now, if the reader will look at plate II., he will see 

 that the base of the stirrup-bone is smooth and flat ; 

 this is placed directly upon the passage into the labyrinth, 

 called foramen ovale, as a seal is placed on an impression, or 

 a valve upon a hole. The cavities we have described are 

 filled with air, those now to be mentioned are filled with wa- 

 ter. The labyrinth then, which has been called the audience 

 chamber of the ear, consists of three parts, viz., the vesti- 

 bule or lobby, which may be compared to the porch of a 

 building ; three semi-circular canals shaped like three hoops 

 tied together at one point, and their opposite points widelj 

 separated ; and lastly, the cochlea, which closely resembler 



