68 



THE CAT, 



[chap. III. 



situated in tlie wall of the petrosal, opposite tlie t3axipanic membrane. 

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

 oralis, the lower and more posterior is the fenestra 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. 1 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. 



J^ 



OiE. 



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 anchyloscd together, which were at first distinct. 



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

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

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

 chamber of the bulla {h) ; 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 \dih parts of 



