374 THE BRAIN 



the posterior part of the inferior surface of the cerebellum, and 

 so open the large cisterna cerebello-medullaris. It lies between 

 the inferior verrnis of the cerebellum and the dorsal surface of 

 the medulla oblongata, and it communicates, through the thin 

 dorsal wall of the medulla oblongata, with the cavity of the 

 hind-brain, which is called fourth ventricle. Note that a large 

 branch of each vertebral artery, called the posterior inferior 

 cerebellar, passes into the cerebello-medullary cistern, on its 

 way to its distribution to the posterior part of the inferior aspect 

 of the cerebellum. 



Turn again to the lower surface of the pons, and carry the 

 median incision in the arachnoid, forwards, into the inter- 

 peduncular region, as far as the posterior border of the in- 

 fundibulum, and so open the cisterna interpeduncularis. Note 

 that the arachnoid which forms the floor or inferior wall of the 

 interpeduncular cistern is perforated posteriorly, on each side, 

 by the oculo-motor nerve, and anteriorly and more laterally by 

 the internal carotid artery. 



Take away the arachnoid which forms the lower wall of the 

 cisterna interpeduncularis, and so expose the basilar artery as 

 it terminates in its two posterior cerebral branches. Find also 

 the two posterior communicating arteries, which run forwards, 

 one on each side, from the corresponding posterior cerebral 

 artery, to join the internal carotid arteries, which enter the 

 antero-lateral angles of the cisterna interpeduncularis. 



Draw the optic chiasma carefully backwards and cut through 

 the arachnoid immediately in front of it, to open the cisterna 

 chiasmatis. Carry the incision in the arachnoid of the cisterna 

 chiasmatis laterally, round the lateral borders of the optic 

 chiasma, and note that the cisterna chiasmatis communicates, 

 round the margins of the chiasma, with the cisterna inter- 

 peduncularis. Take away the arachnoid which has already 

 been divided, and note that beyond the lateral borders of the 

 optic chiasma, both the cisterna interpeduncularis and the 

 cisterna chiasmatis are prolonged laterally, on each side, between 

 the frontal and the temporal regions of the brain, into the stem 

 of the lateral fissure. 



Clean the internal carotid arteries as they lie at the sides of 

 the optic chiasma, and note (i) that each communicates with 

 the corresponding posterior cerebral artery by means of the 

 posterior communicating artery ; (2) the division of each 

 internal carotid into a middle and an anterior cerebral branch. 

 The middle cerebral branch runs laterally into the stem of the 

 lateral fissure, and the anterior cerebral turns medially above the 

 optic chiasma, in the cisterna chiasmatis, to reach the longi- 

 tudinal fissure, into which it passes ; but as it enters the fissure 

 it is connected with its fellow of the opposite side by the anterior 

 communicating artery. 



It is not advisable to follow the cerebral arteries further 

 at this stage. 



Granulationes Arachnoideales (O.T. Pacchionian Bodies). 

 The connection of the arachnoideal granulations with the 

 arachnoid has been referred to already (p. 100). 



