196 A MANUAL OF ANATOMY. 



brain, or the crura cerebri. They are joined together at 

 their line of contact along the middle line. 



A short distance posteriorly the crura are crossed by the 

 transverse fibres of the pons, which pass across the crura 

 to connect the two halves of the cerebellum and form its 

 middle peduncles. 



Along the middle line the pons is marked by a shallow 

 groove in which rests the basilar artery. (The pons really 

 does not bear upon the artery at all, but upon the cerebro- 

 spinal fluid within the subarachnoid spaces, which form, as 

 stated, a " water-bed " for its support.) 



The longitudinal fibres issue from the posterior part of the 

 pons, and from this point to the margin of the foramen mag- 

 num constitute the medulla. 



A full description of these structures will appear later. 



The Central Parts at the Base of the Cerebrum. Begin- 

 ning in front, in the anterior part of the great longitudinal 

 fissure which separates the frontal lobes the under surface 

 of the anterior part of the corpus callosum will be seen 

 (this is the rostrum). 



The extension of the floor of the cerebrum backward from 

 the corpus callosum to the crossing of the optic nerves is in 

 the shape of a thin lamella of grey matter called the lamina 

 cinerea. The lamina cinerea becomes continuous lat- 

 erally with the anterior perforated spaces, and posteriorly 

 with the tuber cinereum. On either sides of the lamina 

 cinerea and in the angle between the temporal and frontal 

 lobes are the anterior perforated spaces, small areas of the 

 base, where the small arteries of the anterolateral gangli- 

 onic systems enter the corpus striatum. The anterior per- 

 forated space is really the under surface of the lenticular 

 nucleus of the corpus striatum. 



From the sides of the corpus callosum a narrow band of 



