THE TEMPORAL BONES. 



67 



numerous eminences and depressions for the convolutions of the cerebrum, and 

 two well-marked grooves for the branches of the middle meningeal artery. 



Borders. The superior border is thin, bevelled at the expense of the internal 

 surface, so as to overlap the lower border of the parietal bone, forming the squam- 

 ous suture. The anterior inferior border is thick, serrated, and bevelled, alter- 

 nately at the expense of the inner and outer surfaces, for articulation with the 

 great wing of the sphenoid. 



The Mastoid Portion (fmaro^, a nipple or teat) is situated at the posterior part of 

 the bone ; its outer surface is rough, and gives attachment to the Occipito-frontalis 

 and Retrahens aurem muscles. It is perforated by numerous foramina; one of 

 these, of large size, situated at the posterior border of the bone, is termed the 

 mastoid foramen; it transmits a vein to the lateral sinus and a small artery from 

 the occipital to supply the dura mater. The position and size of this foramen 

 are very variable. It is not always present; sometimes it is situated in the 

 occipital bone or in the suture between the temporal and the occipital. The 

 mastoid portion is continued below into a conical projection, the mastoid process, 

 the size and form of which vary somewhat. This process serves for the attachment 

 of the Sterno-mastoid, Splenius capitis, and Trachelo-mastoid muscles. On the 

 inner side of the mastoid process is a deep groove, the digastric fossa, for the 

 attachment of the Digastric muscle ; and, running parallel with it, but more in- 

 ternal, the occipital groove, which lodges the occipital artery. The internal surface 

 of the mastoid portion presents a deep, curved groove, the fossa sigmoidea, which 

 lodges part of the lateral sinus ; and into it may be seen opening the mastoid fora- 



Malleus. 

 Incus. 



Tensor tympani. 

 Proc. cochleariformis. 



tube. 



Carotid canal. 



Carotid canal. 

 Fenestra rotunda. 



Pyramid. 



fityloid process. 



Aqueductus Fallopii. 

 Marrow cells. 



FIG. 33. Section through the petrous and mastoid portions of the temporal bone, showing the communi- 

 cation of the cavity of the tympanum with the mastoid antrum. 



men. The groove for the lateral sinus is separated from the innermost of the 

 mastoid air-cells by only a thin lamina of bone, and even this may be partly 

 deficient. A section of the mastoid process shows it to be hollowed out into a 

 number of cellular spaces, communicating with each other, called the mastoid 

 cells, which exhibit the greatest possible variety as to their size and number. At 

 the upper and front part of the bone these cells are large and irregular, and con- 

 tain air. They diminish in size toward the lower part of the bone, those situated 

 at the apex of the mastoid process being quite small and usually containing 

 marrow. Occasionally they are entirely absent, and the mastoid is solid through- 

 out. In addition to these may be seen a large irregular cavity (Fig. 33), 



