210 HEAD AND NECK 



the middle meningeal artery are visible. Posterior to the 

 middle fossaliesthe tentorium cerebelli covering the cerebellum. 

 The peripheral margin of the tentorium is attached, on each 

 side, to the posterior clinoid process, the upper margin of the 

 petrous part of the temporal bone, the posterior inferior angle 

 of the parietal bone, and to the transverse ridge on the inner 

 surface of the occipital bone. The central or free margin 

 crosses the attached margin behind the posterior clinoid 

 process on each side, and is attached anteriorly to the apex 

 of the anterior clinoid process. It bounds an oval opening, 

 the door of the tent, through which pass the midbrain sur- 

 rounded by the arachnoid and the pia mater, and the posterior 

 cerebral arteries. Piercing the midbrain nearer its posterior 

 than its anterior border is the aquseductus cerebri (O.T. 

 aqueduct of Sylvius). Posterior to the aqueduct is the lamina 

 quadrigemina of the midbrain and anterior to it the pedun- 

 culi (O.T. crura) cerebri. Each peduncle consists of an 

 anterior part, the basis pedunculi (O.T. crusta\ and a posterior 

 part, the tegmentum, the two being separated by a lamina of 

 dark coloured tissue, the substantia nigra. The bases pedun- 

 culi are entirely free from each other, but the tegmental 

 portions are united together anterior to the aqueduct. 



Running anteriorly and laterally from the medial side of 

 each peduncle to the angle between the anterior ends of the free 

 and the attached borders of the tentorium, is the third cerebral 

 nerve. Close to the midbrain the nerve passes between the 

 posterior cerebral artery above and the superior cerebellar 

 artery below; and between the free and attached borders 

 of the tentorium it pierces the dura mater, in the middle 

 fossa, and enters the wall of the cavernous sinus. Between 

 the posterior ends of the third nerves lies the upper end of 

 the basilar artery, dividing into the two posterior cerebral 

 branches; and the dissectors should note that the arteries 

 lie in an enlargement of the subarachnoid space which is 

 known as the dsterna inter peduncularis. In the median plane 

 posterior to the midbrain is the divided vena cerebri magna 

 (O.T. great vein of Galen). It passes posteriorly and upwards, 

 and pierces the apex of the tentorium to enter the straight 

 sinus, which lies in the angle of union between the falx 

 cerebri and the tentorium cerebelli. 



Curving posteriorly around the midbrain and ending 

 posteriorly in the great cerebral vein on each side is the vena 



