30 TEMPORAL BONE. 



continued horizontally inwards from the tubercle a rounded emi- 

 nence, the eminentia articularis. The process of bone which is 

 continued from the tubercle of the zygoma into the eminentia articu- 

 laris is the inferior root of the zygoma. The superior root is con- 

 tinued 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. The internal sur- 

 face of the squamous portion is marked by several shallow fossae, 

 which correspond with the convolutions of the cerebrum, and by a 

 furrow for 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 arti- 

 culate 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 fora- 

 mina, 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 ex- 

 panded, the mastoid 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 superior and middle roots of 

 the zygoma, is the large oval opening of the meatus auditorium 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 (fossa 

 sigmoidea) 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 meatus auditorius extemus 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 tympa- 

 num 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 



