THE SKULL THE CEANIUM 



243 



is upon the posterior surface. The opening of the aquseductus vestibuli 

 varies in position and in size ; in some specimens the canal is not 

 pervious. 



The anterior surface of the petrous may be exceedingly narrow. 

 In a large proportion of specimens it is marked by a groove which 

 runs parallel with the anterior border and not far from it. 



The inferior surface exhibits variations in the size and shape of the 

 promontory. In some specimens the fenestra ovalis is almost as large 

 as the fenestra rotunda ; in others the fenestra rotunda is greatly 

 enlarged ; it is subject also to slight variations in the direction of its 



plane. 



HUMAN TEMPORAL BONE. 



If the student has carefully followed the description of the temporal 

 of the cat, the features of the corresponding human bone can be easily 

 recognized ; indeed, it is doubtful if the structure of the human temporal 

 can be properly mastered and remembered without just such detailed 

 work as has been given on a more generalized type, a type not less 

 complicated, but showing clearly parts which in man are rudimentary 

 or have suffered displacement in the antero-posterior compression of 

 the base of the skull as a result 

 of the relative overgrowth of the 

 cranial vault. 



The human temporal differs 

 from a typical mammalian temporal 

 in the following very obvious char- 

 acters : The zygomatic process is 

 relatively smaller ; the mastoid por- 

 tion of the bone is relatively larger 

 (Fig. 175) ; the entotympanic part 

 of the auditory bulla is absent, 



*/ 



thus exposing on the base of the 

 skull much of the inferior surface 

 of the petrous (Fig. 177), while 

 the ectotympanic part of the bulla 

 enclosing the tympanum is a curved 

 compressed tube, the external con- 

 cave wall of which is separated from the glenoid cavity by the 

 Glaseriau fissure and is called the tympanic plate. The two proxi- 



FIG. 175. 



EXTERNAL VIEW OF THE TEMPORAL BONE. 

 OF THE RIGHT SIDE. 



1, squamous portion ; 2, niastoid portion ; 3, apex 

 of the petrous portion; 4, zygomatic process; 5, 

 glenoid fossa; 6, preglenoid tubercle; 7, glenoid 

 fissure ; 8, mastoid process ; 9, back part of digastric 

 fossa; 10, mastoid foramen; 11, tympanic plate 

 forming the vaginal and auditory processes; 12, 

 auditory meatus; lo, styloid process; 14, slight 

 impression of the deep temporal artery. 



