TEMPORAL BONE. 65 



the posterior branch of the arteria meningea media. 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 articulate 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 por- 

 tion : 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 unfrequently situated in the occipital bone. The inferior 

 part of this portion-is round and expanded, the mastoid process, and ex- 

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

 of hearing. In front of the mastoid process, and between the superior 

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

 auditorius externus, surrounded by a rough lip, the processus auditorius. 

 Directly to the inner side of, and partly concealed by the mastoid process, 

 is a deep groove, the digastric fossa ; and a little more internally the oc- 

 cipital groove, which lodges the occipital artery. Upon its internal sur- 

 face the mastoid portion presents a broad and shallow groove (fossa sig- 

 moidea) for the lateral sinus, and terminating in this groove the internal 

 opening of the mastoid foramen. The superior border of the mastoid por- 

 tion is dentated ; and its posterior border, thick and less serrated, articu- 

 lates with the inferior border of the occipital bone. 



The meatus auditorius externus is a slightly curved canal, somewhat 

 more than half an inch in length, longer along its lower than its upper 

 wall, and directed obliquely inwards and forwards. The canal is narrower 

 at the middle than at each extremity, is broadest in its horizontal diameter, 

 and terminates upon the outer wall of the tympanum by an abrupt oval 

 border. Within the margin of this border is a groove for the insertion of 

 the membrana tympani. 



The Petrous portion of the temporal bone is named from its extreme 

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

 forwards into the base of the skull, the base being applied against the in- 

 ternal surface of the squamous and mastoid portions, and the apex being 

 received into the triangular interval between the spinous process of the 

 sphenoid and the basilar process of the occipital bone. For convenience 

 of description it is divisible into three surfaces anterior, posterior, and 

 basilar ; and three borders superior, anterior, and posterior. 



Surfaces. The anterior surface, forming the posterior boundary of the 

 middle fossa of the interior of the base of the skull, presents for exami- 

 nation from base to apex, first, an eminence caused by the projection of the 

 perpendicular semicircular canal ; next, a groove leading to an irregular 

 oblique opening, the hiatus Fallopii, for the transmission of the petrosal 

 branch of the Vidian nerve ; thirdly, another and smaller oblique foramen, 

 immediately beneath the preceding, for the passage of the nervus petrosus 

 superficial minor, a branch of Jacobson's nerve ; and, lastly, a large 

 foramen near the apex of the bone, the termination of the carotid 

 canal. 



6* E 



