THE TYMPANUM. 43d 



on its internal face, but the radiated arrangement is not so dis- 

 tinct. Caldani considers it as formed by filaments, decussating 

 each other at right angles, and intermixed with blood vessels.* 

 The internal layer is a continuation of the lining membrane of 

 the tympanum; it is separated with some difficulty, owirjg to 

 its tenuity. 



The proper membrane of the tympanum, when successfully 

 injected, exhibits a high degree of vascularity ;f though, in its 

 natural state, but very few red blood vessels are seen in it. 



The floor of the tympanum, or the side next to the labyrinth, 

 presents an unequal surface. In its middle is a well marked 

 rising, the Promontory, (Promontorium,) formed by one end of 

 the labyrinth. Just above the superior margin of this promi- 

 nence, near its centre, is an oval opening, called, from its shape, 

 Foramen Ovale, or Fenestra Ovalis; having its long diameter 

 horizontal, its superior margin rounded or concave, and its infe- 

 rior straight. At the posterior inferior part of the promontory 

 is another opening, which, though somewhat triangular, is called 

 the Foramen Rotundum,or Fenestra Rotunda, and in the dried 

 bone leads to the cochlea, but is naturally stopped by the lining 

 membrane of the tympanum. According to M. Ribes, this 

 membranous plug has also two other layers, an internal one, the 

 continuation of what lines the cochlea, and a middle one, which 

 is peculiar. In these respects, there is a correspondence with 

 the membrana tympani. 



The Eminentia Pyramidalis is a small conical eminence pro- 

 jecting from the posterior part of the tympanum, on a line with 

 the fenestra ovalis. It is hollow, contains a muscle, and com- 

 municates at the other end with the Canal of Faltopius. Lower 

 down, and more externally, there is a small orifice, (Apertura 

 Chordce,} through which the nerve called Chorda Tympani 

 passes. 



The Mastoid Portion of the Temporal bone, in the adult, 

 abounds in large cells or sinuses, which communicate freely 



* Plate XCVIII. Anat. 



| Ruyschii, Epist. Anat. Probl. viii. Anatomical Museum. 



