34 THE SKELETON Of-' THE CAT. 



mastoid portion (Figs. 22 and 23, e), which is flattened and 



triangular and is attached by its base to the base of the 



pyramid. 



The petrous portion may be described as having a base and 



three sides, lateral, dorsal, and medial. It completes the 



medial wall of the tympanic bulla, so 

 that it is not possible to see it from the 

 exterior of a skull except through the 

 auditory meatus (Fig. 22, z). When the 

 bones of the skull are articulated its dor- 

 sal surface is covered by the tentorium 

 , b and alisphenoid. Its lateral face looks 



FIG. 25. PETROUS BONE OF into the tympanic cavity, while the 



RIGHT SIDE, LATERAL me dial face looks into the cranial cavity. 

 SURFACE, ENLARGED. 



a, fenestra cochlese; 6, pro- Its lateral face (Fig. 25) (medial 



montory; f, fenestra vesti- wa n o f the tympanic cavity) presents 



buli; d, fossa for the tensor . -,11 



tympani muscle; e, fossa for just ventrad of the middle of its base a 

 incus and malleus; /, fossa large c i rcu i ar foramen, the fenestra 



continuous with stylomastom 



foramen; g, foramen leading cochleae (a) (or fenestra rotunda), which 

 to facial canal. looks caudo i atera d ; it leads into the 



cochlea. The fenestra cochlea; lies at the summit of a nipple- 

 like elevation, the promontory ($), which is continued toward 

 the apex of the bone as a gradually diminishing semicylindrical 

 ridge, due to the presence within it of the bony cochlea. 

 Dorsad of the fenestra cochleae is the much smaller fenestra 

 vestibuli (r) (or ovalis) which leads into the vestibule. It is 

 occupied in the natural condition by the base of the stapes. 



Dorsocraniad of the fenestra vestibuli is a large fossa (</) 

 which contains the tensor tympani muscle. Dorsocaudad of 

 this, partly bounded by the squamous portion of the temporal, 

 is another large fossa (*'), the cranial end of which is occupied 

 by the incus, while its caudal end is occupied by the head of 

 the malleus. 



Nearly caudad of this fossa and separated from it by an 

 oblique bony septum is a third fossa (f) which is narrow and 

 curved. It is continuous with a notch in the mastoid portion 

 of the bone. When the tympanic is articulated the notch is 

 converted into a foramen (stylomastoid foramen, Fig. 22,7) 



