68 



THE CAT. 



[CHAP. in. 



situated in the wall of tlie petrosal, opposite the tympanic membrane. 

 The upper, more anterior, and smaller of these is called the fenestra 

 ovalis, the lower and more posterior is the fenesira rotunda. It is 

 through these that the really internal ear (a complex membrane 

 which is lodged in a correspondingly complex bony envelope within 

 the petrosal) is placed in communication with the exterior (Fig. ] 37). 



The internal, posterior, and much larger chamber, is entirely closed, 

 save that a small opening (Fig. 36,*) is left between the hinder part of 

 the top of the septum close to the fenestra rotunda, so that this fenestra 

 may be said to open into the inner as well as the outer chamber. 



In front of the fenestra rotunda is a bony prominence, caused by 

 the presence within it of a part of the internal ear called the cochlea. 



.A. 



Fig. 37. THE SPHENOID. 



A. Under surface. 



B. Upper surface. 



oe. Anterior clinoid process. 

 *. Ali-sphenoid. 

 Is. Basi-sphenoid. 

 c.p. Clinoid plate. 

 lip. Hamular process, 

 os. Orbito-sphenoid. 

 p. Ethmoidal process. 

 pf. Pterygoid fossa. 



ps. Pre-splienoid. 

 pt. Pterygoid plate. 

 r. Rostrum. 



7. Optic foramen. 



8. Foramen rotundum. 



9. Foramen ovale. 



The anterior and posterior portions are repre- 

 sented as separated, and thus the sphenoidal 

 fissure (which is defined by their junction) 

 is not indicated. 



This prominence is called the " promontory " (pt). Above it, and in 

 front of the fenestra ovalis, is a pit, or fossa, within which arises a 

 small muscle called the Stapedius. 



The temporal bone, as has been said, is really made up of several 

 bones anchylosed together, which were at first distinct. 



Thus we have (1) the squamosal (sq), with its zygomatic 

 process (2) ; (2) the tympanic, which forms the outer chamber of 

 the bulla (t) ; and (3) the entotympanic, which forms the inner 

 chamber of the bulla (b) ; and (4) the minute tympano-hyal. 

 Besides these four elements three other distinct ossifications extend 

 and coalesce to form the petrous and mastoid portions of the temporal 

 bone, and are distinguished by their diverse relations with parts of 



