TEMPORAL BONE. 37 



eminentia articularis is the inferior root of the zygoma. The supe- 

 rior root is continued upwards from the upper border of the zygoma, 

 and forms the posterior part of the temporal ridge, serving by its 

 projection to mark the division of the squamous from the mastoid 

 portion of the bone ; and the middle root is continued directly 

 backwards, and terminates abruptly at a narrow fissure the 

 fissura Glaseri or glenoid fissure. The internal surface of the squa- 

 mous portion is marked by several shallow fossas, which correspond 

 with the convolutions of the cerebrum, and by a furrow for the pos- 

 terior branch of the arteria meningea magna. The superior or 

 squamous border, is very thin and bevelled at the expense of the 

 inner surface, so as to overlap the lower and arched border of the 

 parietal bone. The inferior border is thick and dentated to articu- 

 late with the spinous process of the sphenoid bone. 



The Mastoid portion forms the posterior part of the bone ; it is 

 thick and hollowed between its tables into a loose and cellular diploe. 

 Upon its external surface it is rough for the attachment of muscles, 

 and contrasts strongly with the smooth and polished-like surface of 

 the squamous portion ; every part of this surface is pierced by 

 small foramina, which give passage to minute arteries and veins ; 

 one of these openings, oblique in its direction, of large size, and 

 situated near the posterior border of the bone, the mastoid foramen, 

 transmits a vein to the lateral sinus. This foramen is not unfre- 

 quently situated in the occipital bone. The inferior part of this por- 

 tion is round and expanded, the m.astoid process, and excavated in 

 its interior into numerous cells, which form a part of the organ of 

 hearing. In front of the mastoid process and between the supe- 

 rior and middle roots of the zygoma, is the large oval opening of the 

 mealus audilorius externus, surrounded by a rough lip, the processus 

 auditorius. Directly to the inner side, and partly concealed by the 

 mastoid process, is a deep groove, the digastric fossa ; and a little 

 more internally the occipital groove, which lodges the occipital 

 artery. Upon its internal surface the mastoid portion presents a 

 broad and shallow groove for the lateral sinus, and terminating in 

 this groove the internal opening of the mastoid foramen. The 

 superior border of the mastoid portion is dentated, and its posterior 

 border thick and less serrated for articulation with the inferior border 

 of the occipital bone. 



The Petrous portion of the temporal bone is named from its ex- 

 treme hardness and density. It is a three-sided pyramid, projecting 

 horizontally forwards into the base of the skull, the base being 



tion. 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 eminentia 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 sty- 

 loid process. 12. The vaginal process. 13. The glenoid or Glaserian fissure; the 

 leading line from this number crosses the rough posterior portion of the glenoid fossa. 

 1 4. The opening and part of the groove for the Eustachian tube. 



