BLOOD VESSELS OF THE BRAIN 379 



Veins of the Pons. The veins from the upper part of the 

 pons join the basal vein, and those from the inferior part 

 either join the cerebellar veins or they end in the inferior 

 petrosal sinuses. 



Veins of the Medulla Oblongata. The smaller veins of 

 the medulla oblongata converge to an anterior and a posterior 

 median vein, or they run along the roots of the last four 

 pairs of cerebral nerves. The anterior median vein com- 

 municates, above, with the veins of the pons, and, below, 

 with the veins of the spinal medulla. The posterior median 

 vein also communicates, below, with the veins of the spinal 

 medulla, and it terminates, above, either in the inferior 

 petrosal sinuses or in the basilar plexus. The efferents 

 which accompany the last four cerebral nerves end either in 

 the inferior petrosal sinuses, in the upper parts of the internal 

 jugular veins, or in the pharyngeal plexus. 



Arteries which supply Blood to the Brain. Four main 

 arterial trunks carry blood into the cranium for the supply 

 of the brain viz., the two internal carotid arteries and the 

 two vertebral arteries. The vertebral arteries enter through 

 the foramen magnum, whilst the internal carotid arteries gain 

 admittance through the lacerate foramina, after traversing the 

 carotid canals. Both the vertebral and the internal carotid 

 arteries were divided when the brain was removed from the 

 cranium. The cut ends of the internal carotids will be seen, 

 at the base of the brain, close to the sides of the optic 

 chiasma. When the remains of the membranes are taken 

 away from around it, each internal carotid will be found to 

 divide, a short distance above its cut extremity and immedi- 

 ately below the anterior perforated substance, into a larger 

 branch, the middle cerebral artery, which runs laterally, and 

 a smaller branch, the anterior cerebral artery, which runs 

 medially. 



The vertebral arteries curve round the sides of the medulla 

 oblongata and they unite at its upper border, in the median 

 plane, to form the basilar artery, which runs to the upper 

 border of the pons, where it divides into the two posterior 

 cerebral arteries. 



But the cerebral arteries which spring from the internal 

 carotid arteries of opposite sides are brought into association 

 with one another, and with the posterior cerebral arteries, 

 which spring from the basilar, by a remarkable and complete 



