THE SKULL THE CRANIUM 179 



temporal. At a variable point on the lower margin is the beginning 

 of a vertical arborescent groove for the meningeal artery. The 

 groove is a continuation of the groove on the anterior surface of 

 the tentorium. 



The Tentorium or tentorial process is a plate of bone directed 

 downward and forward and also inward at an angle of forty-five 

 degrees from the posterior border of the inner surface. It is about 

 one-third as large as the bone itself. 



Its inner border is straight and joins the process on the other 

 parietal ; its outer border is in part the posterior half of the inferior 

 border of the bone, in part independent and short, joining the inner 

 surface of the squamous part of the temporal. The anterior or lower 

 border consists of a lower and an upper part. The lower part, com- 

 posing about one-third of the border, is transverse, slightly ^marginate, 

 and exhibits a prominent inner angle. It is jagged and bevelled 

 at the expense of the posterior surface, and it articulates with the 

 alisphenoid by overlapping its tentorial process. In its upper two- 

 thirds the border is deeply emarginate and the upper angle is ap- 

 parently produced downward. This part of the border is rounded, or 

 slightly swollen, and affords attachment to a membrane of the brain 

 called the dura mater. 



The upper surface of the tentorium is almost flat, and is marked 

 by some irregular depressions for cerebral convolutions and by occa- 

 sional openings and grooves for cerebral vessels. Of these grooves, 

 the one beginning at the inner lower angle and running obliquely 

 upw r ard and outward is very constant. The upper surface faces for- 

 ward, outward, and upward, and supports the lower surface of the 

 middle lobe of the brain. It turns forward above into the internal 

 surface of the bone. 



The lower surface of the tentorium faces downward, backward, 

 and inward. It shows three distinct portions : (1) an upper, median 

 portion, which is quadrate, slightly concave, faces inward more than 

 the other portions of the surface, and covers part of the cerebellum 

 (Fig. 118). (2) External to this portion, and separated from it by a 

 longitudinal ridge, is the second portion, which forms a large triangular 

 area, also concave, covering the lateral part of the cerebellum. It is 

 marked by faint secondary fossa?, and exhibits on its posterior border 

 a groove and a foramen for the lateral sinus, (o) Below and in front 



