io8 HEAD AND NECK 



In the dura mater, on each side of the hypophyseal (pituitary) 

 fossa, lies the corresponding cavernous sinus, which will be 

 dissected later, and still more laterally are the depressed lateral 

 portions of the middle cranial fossa, lined with dura mater, in 

 which the trunk and some of the branches of the middle 

 meningeal artery are visible. Posterior to the middle fossa lies 

 the 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 mastoid 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, 

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

 cerebral arteries. Traversing the midbrain, nearer its posterior 

 than its anterior border, is a canal called the aquaeductus cerebri 

 (O.T. aqueduct of Sylvius). Posterior to the aqueduct is the 

 lamina quadrigemina or tectum of the midbrain, and anterior 

 to it are the right and left pedunculi (O.T. crura) cerebri. Each 

 peduncle consists of an anterior part, the basis pedunculi (O.T. 

 crusla), and a posterior part, the tegmentum, the two being 

 separated by a lamina of dark coloured tissue, the substantia 

 nigra. The bases pedunculi are entirely free from each other, 

 but the tegmental portions are united together, anterior to the 

 aqueduct. 



Running forwards 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 oculomotor 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 

 oculomotor nerves lies the upper end of the basilar artery, divid- 

 ing into the two posterior cerebral branches ; and the dissectors 

 should note that the arteries lie in an enlargement of the sub- 

 arachnoid space which is known as the cislerna interpeduncularis. 

 In the median plane, posterior to the midbrain, is the divided vena 

 cerebri magna (O.T. great vein of Galen}. It passes backwards 

 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 backwards around the midbrain and ending 

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

 basalis, and immediately above it, running forwards, is the 

 slender trochlear nerve. If the free border of the tentorium is 

 turned laterally, at the point where it is crossing the attached 

 border, the trochlear nerve will be seen perforating the inner 

 layer of the dura mater to enter the wall of the cavernous sinus. 



When the dissectors have verified the facts noted above, 

 they should examine the lower, free border of the falx cerebri, 

 in which they will find the small inferior sagittal sinus, which 

 terminates posteriorly, at the apex of the tentorium, in the straight 

 sinus. The straight sinus must now be opened by carrying the 



