550 HEARING. 



lique projection, called the promontory ; above this projection 

 there is an opening of the labyrinth called thefenestra rotunda' 1 , 

 and which is shut by a membrane ; below, there is another aper- 

 ture of the labyrinth, thefenestra ovalis, where the impressions of 

 the vibrations upon the membrana tympani are communicated to 

 the labyrinth, by a very flexible intermediate apparatus, consist- 

 ing of a chain of jour small bones k , the malleus or hammer, the 

 incus or anvil, the lenticular bone, and the stapes or stirrup. 



The hammer consists of a long and thin handle, the extremity 

 of which adheres to the membrane of the tympanum ; and of a 

 head, which forms an angle with the handle and is articulated with 

 the anvil. It is united to the bony rim of the membrane by its 

 spinous process, which may be regarded as the fixed point of the 

 lever. The anvil is on one side articulated with the head of the 

 hammer ; and the opposite part has two processes, one of which 

 serves as a resting point, and the other is articulated by means of 

 the lenticular bone with the stirrup. The stirrup, which is so 

 called from its close resemblance in form to that object, makes 

 almost a right angle with the anvil, and its mov cable base closes the 

 fenestris ovalis of the labyrinth, the interior of which it agitates 



1 . External ear. 



2. A part of it called concha ; and orifice 



of the meatus externus. 



3. Meatus externus. 



4. Membrana tympani. 



5. Malleus. 



6. Incus. 



7. Stapes and os lenticulare. 



8. Vestibule. 



9. Three semicircular canals. 

 10. Cochlea. 



1 " Scarpa, De Structura Fenestrte Rotunda;, &c. Mutin. 1772. 8vo." 

 k " The existence of a fourth bone (called lenticular), commonly admitted 

 since the time of Franc. Sylvius, I have disproved at large in my Osteology, 

 p. 155. sq. edit. 2. It is wanting in the greater number of perfect examples 

 from adults." 



