198 



MAMMALIAN ANATOMY 



FIG. 133. 



THE PETROUS PORTION. 



The petrous portion (Fig. 133) is easily distinguished by its dense, 

 white, glistening appearance, and is free from the other portions except 



at its posterior outer part, or base, where 

 it cannot be readily separated from the 

 squamons and mastoid. It is concealed 

 on the base of the skull by the auditory 

 bulla, although a, part of its surface 

 may be seen by looking through the 

 auditory meatus. In the interior of 



/ 



the skull it can be easily recognized 

 lying at the side under the lower end 

 of the tentorium. 



The petrous has the form of a prostrate pyramid, whereof the 

 base faces outward, upward, and backward and the apex points down- 

 ward, forward, and inward. Of the three surfaces the posterior and 

 the inferior are large and pear-shaped and about equal in size, while 



INNER VIEW. 

 POSITION OF PETROUS. 



FIG. 134. 



Sagittal Crest. 



Wall of Cranium. 

 Cut Edge of Tentorium. 



Squamous. 



For Malleus. 



Aquxductus Fallopit. 



Fenestra Ovalis._ 



For Incus. 



Cochlea. 



Inner Chamber of Bulla. 

 Upper Edge of Septum. 



Outer Chamber of Bulla 

 Septum. 



Parietal. 



Tentorium Cerebelli. 



f j) Squamous of Temporal. 



Cells in Petrous. 



' TENSOR TYMPANI. 



f-'cncstra Ontlis. 



Slit for STAPEDIUS. 



^ _ [Chambers of Bulla. 



"i ' Opening of Communication between Two 



R V 



"''' Mastoid of Temporal. 

 Cochlea. 



Posterior Kim of External Auditory Meatut 

 of Ectotympanic of Bulla. 



Ent.otympanic of Bulla. 

 Groove in Basioccipital for Blood-vessel. 



Inner Chamber of Bulla. 

 TRANSVERSE VERTICAL SECTION OF SKULL THROUGH EXTERNAL AUDITORY MEATUS. 



the anterior is only about half as wide as each of the other two. The 

 petrous is so placed that the anterior surface, which is almost entirely 

 covered by the lower part of the under surface of the tentorium, faces 

 forward, upward, and outward ; the posterior surface faces backward, 



