MIDDLE EAR. 95 



in the bone at the inner end of the auditory canal ; its sur- 

 face is convex internally, and concave externally; it is 

 placed at an angle of 45 with the floor of the canal. One 

 of the little bones of the ear, the malleus, is attached to 

 the upper part of its internal surface, which is also crossed 

 by the chorda tympani nerve. In the foetus this membrane 

 is connected with a separate, osseous ring, the tympanic 

 bone, which subsequently unites with the rest of the tem- 

 poral bone. 



* 



MIDDLE EAR. 



The tympanum is behind the tympanic membrane, and is exposed 

 by removing the bone which forms the roof of the cavity, in such a 

 way as to preserve as perfectly as possible the membrane and the 

 lain of little bones within it. 



The TYMPANUM, or MIDDLE EAR, has the form of a round, 

 flat box, placed on edge ; the following points are to be 

 noticed within its cavity. Upon the surface, opposite the 

 membrana tympani, a central projection, called the promon- 

 tory ; above and below this, two apertures opening into the 

 labyrinth ; the superior one is called the fenestra ovalis, 



d the lower the fenestra rotunda; the fenestra rotunda 



closed by a thin membrane, called the secondary mem- 

 brana tympani. The anterior boundary of the tympanic 

 cavity is formed by the membrana tympani and a part of 

 the surrounding bone; above the membrane maybe seen 

 ihefissura Glaseri, occupied by the long process of one of 

 the small bones of the ear, the malleus, and by a small mus- 

 cle, the laxator tympani ; crossing the membrana tympani 

 at its upper part is the chorda tympani nerve. This nerve 

 is a long but slender branch of the facial ; it arises about a 

 quarter of an inch from the stylo-mastoid foramen, and 

 passes forward to the tympanum, entering that cavity just 

 below the pyramid ; it then crosses the handle of the mal- 

 leus and the membrana tympani to a small foramen on the 

 inner side of the Glaserian fissure, through which it passes, 

 and, emerging, unites with the gustatory nerve. 



It is to be noticed that the circumference of the tympanic 

 cavity presents a rough surface, separated superiorly from 

 the cranial cavity, and inferiorly from the jugular fossa, 

 merely by a thin, osseous plate; this is an anatomical fact 

 of importance in connection with fractures of the petrous 

 portion of the temporal bone, which, if at this point, would 

 lead to serious complications. At the posterior part of the 



