THE ARTERIES OF THE BRAIN 361 



(e) The cut end of the internal carotid artery Hes on either 

 side of the tuber cinereum. It turns forward but is con- 

 nected backwards with the posterior cerebral by a posterior 

 communicating artery. 



(/) The middle cerebral artery (a. cerebri media) is given off 

 from the internal carotid, branching over the middle por- 

 tion of the hemisphere to supply most of its lateral and 

 dorsal surfaces. 



(g) The anterior cerebral artery (a. cerebri anterior) is the con- 

 tinuation of the carotid after the origin of the middle 

 cerebral artery. It passes to the anterior portion of the 

 ventral surface of the cerebral hemisphere and to the 

 olfactory bulb. The anterior cerebral unites with that of 

 the other side to form a short common trunk between the 

 hemispheres which redivides into paired vessels supplying 

 the medial surfaces. A complete anastomotic loop is thus 

 formed round the hypothalamus by the internal carotid, 

 anterior cerebral, posterior communicating, and posterior 

 cerebral arteries. This is the circle of Willis. 



The fusion of the anterior cerebral arteries replaces an inter- 

 connection by an anterior communicating artery, which occurs in man 

 and many other mammals and occasionally appears in rabbits as an 

 individual variation. 



6. By dividing the supports of the cerebellum on either side, the 

 entire structure may be removed and the surface may be ex- 

 posed, as in Fig. 122, for an examination of the structures of 

 the dorsal surface of the rhombencephalon. The posterior 

 medullary velum is removed with the cerebellum, but the 

 anterior medullary velum should be cut so that a small portion 

 of it remains in place. 



(a) The fourth ventricle (ventriculus quartus) is the extensive 

 space enclosed by the rhombencephalon. It is connected 

 forwards with the cerebral aqueduct (the cavity in the 

 midbrain) and backwards with the central canal of the 

 spinal cord. Its roof is formed principally by the anterior 

 and posterior medullary vela, these being attached to the 

 cerebellum close to each other so that they underlie it. 



