MEMBRANES AND BLOOD VESSELS 443 



removal of the brain. The cut ends of the internal carotids 

 will be seen, on the base of the brain, immediately to the 

 lateral sides of the optic nerves ; the vertebral arteries lie 

 on the sides of the medulla oblongata. A very remarkable 

 and complete anastomosis takes place at the base of the 

 brain by the inosculation of branches which spring from 

 the internal carotid and vertebral arterial systems. This is 

 termed the circulus arteriosus (Willis), and the branches 

 which take part in its formation lie in the cisterna inter- 

 peduncularis and the cisterna chiasmatis. 



Two systems of branches, both going to the cerebrum but 

 differing greatly in their mode of distribution, proceed from 

 the vessels forming the arterial circle. One system consists 

 of very numerous slender twigs, which, as a rule, come off in 

 groups in certain localities, and at once pierce the substance 

 of the cerebrum so as to gain its interior. These are the 

 central or basal branches. The other system is composed of 

 branches which ramify over the surface of the cerebrum, and 

 is termed the system of cortical branches. The central parts 

 of the brain, including the basal ganglia, receive their blood- 

 supply from the basal system, and the vessels which con- 

 stitute this group do not anastomose with each other. The 

 cortical vessels supply the cerebral cortex and the finer 

 branches, which ramify in the pia mater, anastomose with 

 one another ; therefore, the neighbouring vascular districts of 

 the cerebral cortex are not sharply cut off from each other. 



Arteria Vertebralis. The vertebral artery enters the sub- 

 arachnoid space in the upper part "of the vertebral canal by 

 piercing the dura mater and the arachnoid. Gaining the 

 interior of the cranium, through the foramen magnum, it is 

 continued upwards on the side of the medulla oblongata. 

 Soon it inclines to the anterior aspect of the medulla 

 oblongata, and, meeting its fellow of the opposite side in 

 the median plane, it unites with it, at the lower border of 

 the pons, to form the basilar artery. 



During this part of its course the vertebral artery gives off 

 the following branches : 



1. Posterior spinal. 3. Anterior spinal. 



2. Posterior inferior cerebellar. 4. Bulbar. 



Arteria Spinalis Posterior. This is the first branch that 

 is given off after the vertebral artery pierces the dura mater. 



