68 OSSA TEMPORUM. 



the internal side of its base is a deep groove in which the pos- 

 terior belly of the digastric muscle is inserted. Behind this 

 process is the mastoid hole, which transmits a vein, and some- 

 times a small artery. 



On the internal surface of this portion is a large groove, 

 which is formed by the lateral sinus. The mastoid hole above 

 mentioned, opens into this groove. 



The petrous portion, which is * Fig. 14.* 



situated between the squa- 

 mous and mastoid, resembles 

 a triangular pyramid lying on 

 one of its sides. When in its 

 proper position it projects 

 inward and forward. The 

 two upper sides form a por- 

 tion of the internal surface of 

 the base of the cranium. The 

 angle formed by these sur- 

 faces is very prominent, and divides the fossa for the middle 

 lobes of the brain, or rather the cavities for the cerebrum from 

 those which contain the cerebellum. 



One of these sides of the petrous portion looks forward and 



outward, the other backward and inward. Each of them has 



eminences and depressions to correspond with the convolutions 



i of the brain. Near the middle of the anterior side is a small 



j furrow, leading to a foramen denominated Innominatum or 



i Hiatus Fallopii which transmits the vidian nerve to the aque- 



i duct of Fallopius. 



* The external surface of the temporal bone of the left side. 1. The squamous 

 portion. 2. The mastoid portion. 3. The extremity of the petrous portion. 

 4. The zygoma. 5. Indicates the tubercle of the zygoma, and at the same time 

 its anterior root turning inwards to form the eminenlia articularis. 6. The 

 superior root of the zygoma, forming the posterior part of the temporal ridge. 



7. The middle root of the zygoma terminating abruptly at the glenoid fissure. 



8. The mastoid foramen. 9. The meatus auditorius externus, surrounded by 

 the processus auditorius. 10. The digastric fossa, situated immediately to the 

 inner side of (2) the mastoid process. 11. The styloid process. 12. The vagi- 

 nal process. 13. The glenoid or Glaserian fissure ; the leading line from this 

 number crosses the rough posterior portion of the glenoid fossa. 14. The open- 

 ing and part of the groove for the Eustachian tube. 



