454 CAVITY OF THE TYMPANUM. 



On the inner side of this septum is the cavity of the Tym- 

 panum, which may be regarded as the continuation of the bony 

 meatus. 



It differs, however, from the form of this canal ; for its 

 diameter is greater, and it is not so regularly cylindrical. 



It is not deep ; as the distance from the membrana tympani, 

 which constitutes the external side, to the opposite internal 

 side, is seldom more than three or four lines. 



Its antero-posterior diameter is about six lines, and its 

 vertical diameter rather more: though from the inequality of 

 its surface, and its connexion with the mastoid cells and 

 Eustachian tube, it is difficult to assign its exact dimensions. 



The breadth of the cavity of the tympanum is, therefore, 

 greater than its depth. 



It is situated immediately between the membrana tympani, 

 and the labyrinth which is on the inside of it. 



In its natural state this cavity has two apertures ; one of which 

 communicates with the fauces, by means of the Eustachian 

 Tube ; and another which leads into the cellular structure of the 

 mastoid process. 



There are also two deficiencies in the bony plate, which 

 separates it from the labyrinth, called the Foramen Ovale, 

 and Foramen Rotundum ; but in the natural state of the parts 

 these deficiencies are closed, and there is no direct communica- 

 tion of the tympanum either with the labyrinth or the meatus 

 auditorius externus: the only direct communication being those 

 above mentioned, with the fauces and the mastoid cells. 



That deficiency in the bone between the tympanum, and the 

 labyrinth, which is called Foramen Ovale, is closed by one of 

 the small bones of the ear, called the Stapes. The other defi- 

 ciency, called Foramen Rotundum, is covered by the membrane 

 which lines the tympanum. 



In the cavity of the tympanum are four of the smallest bones 

 of the body, which are articulated with each other so as to form 

 one flexible piece. This piece is attached by one end to the 

 membrana tympani, and the other to the Foramen Ovale ; and 

 it is moved by small muscles connected with it. 



