Method XX' 117 



which, in its turn, opens into the sinus transversus near, and 

 perhaps to the left of, its junction with the sinus sagittalis 

 superior. 



Among the other larger veins may be noted the venae cere- 

 bri mediales, passing from the ventral surface of the brain by 

 way of the fissura cerebri lateralis (fissure of Sylvius). 



2. Arteries. — The arteries of the encephalon are derived 

 from two sources, the two vertebral arteries and the two internal 

 carotids. These approach the brain from its ventral surface. 

 Only the larger arteries will be noted. 



Lay the specimen ventral surface upward and identify the 

 following : The two arteriae vertebrales, along either side of 

 the medulla oblongata, fuse at the posterior border of the pons 

 to form the larger single arteria basalis. The A. basilaris passes 

 along the pons in its sulcus basilaris, giving off the Aa. cere- 

 belli inferiores and superiores a:nd the smaller rami ad pontem. 



When the A. basilaris reaches the anterior border of the 

 pons, it divides into the two Aa. cerebri posteriores which 

 spread out over the occipital portion of the brain. 



Almost immediately after their origin from the A. basilaris, 

 the Aa. cerebri posteriores give off a branch each. These are 

 the Aa. communicantes posteriores which connect the arterial 

 system arising from the vertebral arteries with that arising from 

 the internal carotids. 



Each A. carotis interna is Joined by the A. communicans 

 posterior just at the side of the chiasma opticum. 



The A. carotis interna divides into three branches. The A. 

 choroidea, the smallest of these, passes posteriorly around the 

 pedunculus cerebri (crus), and inward to the ventricles of the 

 brain. The A. cerebri media, the largest branch, passes along 

 the fissura cerebri lateralis (Sylvii), and its branches spread over 

 the lateral aspect of the brain. The third branch of the A. 

 carotis interna is the A. cerebri anterior which supplies the 

 frontal lobes, and is the chief source of supply to the mesial 

 surface of the brain. 



Across the mid-line and just in front of the chiasma opticum. 



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