348 HEARING. LECT. XIX. 



exists a fifth opening, namely, that of the Eustachian tube, 

 which communicates with the upper part of the pharynx. 



Within and across the tympanum is fixed a chain of small 

 bones ; which from the analogy of their forms, are called 

 the hammer (malleus,) the anvil (incus,) the orbicular bone 

 (os orbiculare,) and the stirrup (stapes.) The hammer is 

 attached parallel to the membrane of the tympanum, like a 

 solid ray going from the circumference to the centre 

 of the latter. One extremity touches the anvil, which is 

 connected with the orbicular bone, and the latter with the 

 stirrup, which terminates at the fenestra ovalis. Several 

 muscles effect slight movements in the chain, shortening 

 and lengthening it, and thus varying the degree of tension 

 of the membranes upon which it presses. 



Beyond, or on the inner side of the tympanum, in the 

 substance of the petrous bone, is situated what is called the 

 labyrinth, or internal ear ; formed of different cavities, which 

 communicate with each other, and are distinguished by the 

 names of the vestibule, the semi-circular canals, and the 

 cochlea. The vestibule occupies the central part, and com- 

 municates with the tympanum by the fenestra ovalis, and 

 with the three semicircular canals, which are expanded at 

 their extremities, forming ampulla?. The cochlea, so-called 

 because it is curved spirally, like the shell of a snail, com- 

 municates with the interior of the vestibule, and is sepa- 

 rated from the tympanum by the fenestra rotunda. The 

 vestibule and the cochlea contain a fluid called the liquor 

 of Cotunnius, in which float the filaments of the acoustic 

 nerve. Such is a succinct enumeration of the principal 

 parts of the ear of man, and the higher animals. We shall 

 examine the propagation of the sonorous waves through 

 these parts, in order to deduce therefrom the physical 

 theory of hearing. 



Propagation of Sound through the jEr.- The ear is com- 



