HEARING. 



425 



this reason that we instinctively open the mouth 

 when we are intent on hearing a very faint or 

 distant sound. 



On the side of the cavity of the tympanum, 

 which is opposite to the opening of the Eu- 

 stachian tube, is situated the beginning of 

 another passage, leading into numerous cells, 

 contained in the mastoid process of the temporal 

 bone, and therefore termed the mastoid cells: 

 these cells are likewise filled with air. The 

 innermost side of the same cavity, that is the 

 side opposite to the ear-drum, and which is 

 shown in Fig. 391, is occupied by a rounded 

 eminence (p), of a triangular shape, termed the 

 "promontory; on each side of which there is an 



opening in the bone, closed, however, by the 

 membrane lining the whole internal surface of 

 the cavity. The opening (o), which is situated 

 at the upper edge of the promontory, is called 

 the fenestra oralis, or oval window ; and that 

 near the under edge (r), is i\\e fenestra rotunda^ 

 or round window. 



Connected with the membrane of the ear- 



