724 



DISSECTION OF THE BKAIN. 



Anterior 



choroid 



artery. 



Circle of 

 Willis : 



vessels that 

 form it. 



Use of the 

 free in- ' 

 osculation. 



Other 



anastomoses 

 are small. 

 Veins of the 

 brain. 



Two sets to 

 cerebrum : 



external, 

 which are 

 upper and 

 lower ; 



and internal. 



V.-iii.s of 

 cerebellum. 



Dissection. 



'Care to be 

 taken in 

 removing 

 ]>ia mater. 



cortical distribution of the anterior cerebral branch of the internal 

 carotid is mainly on the mesial, whilst that of the middle cerebral is 

 on the outer surface of the cerebral hemisphere. 



The anterior choroid artery (fig. 257, 5 and fig. 258), is small, and 

 arises either from the trunk of the carotid, or from the middle cerebral 

 artery : it passes backwards on the outer side of the posterior com- 

 municating artery, and makes its way between the hemisphere and 

 the cms cerebri into the dentate fissure, at the bottom of which it 

 enters the choroid plexus of the lateral ventricle. 



CIRCLE OF WILLIS (fig. 257). The arteries at the under part of 

 the brain are united freely both on their own side and across the 

 middle line in an anastomotic ring the circle of Willis. On each 

 side this ring is formed by the trunk of the internal carotid giving 

 forwards the anterior cerebral, and backwards the posterior com- 

 municating artery. In front it is constructed by the converging 

 anterior cerebrals, and the anterior communicating artery. And 

 behind is the bifurcation of the basilar trunk into the posterior 

 cerebrals which receive the posterior communicating. In the urea of 

 the circle lie several parts of the brain corresponding with the floor 

 of the third ventricle. 



The complete inosculation between the cranial vessels in the circle 

 of Willis possibly allows at all times a free circulation of blood 

 through the brain, even though a large vessel on one side of the neck 

 should be obstructed. 



Beyond the circle of Willis the arteries of the cerebrum communi- 

 cate together only by fine anastomoses. 



The VEINS of the brain enter the sinuses of the dura mater, and 

 do not form companion trunks to the arteries. 



Two sets of veins belong to the cerebrum, vix., superficial or 

 external, and deep or internal. 



The superficial veins of the upper part of the hemisphere ascend 

 to the superior longitudinal sinus ; and those of the lateral and 

 under parts enter the sinuses in the base of the skull, especially 

 the cavernous and lateral sinuses. These vessels communicate freely 

 together. 



The deep veins of the cerebrum join the veins of Galen (p. 764), 

 and reach the straight sinus. 



The veins of the cerebellum end differently above and below. On 

 the upper surface they are received by the veins of Galen and the 

 straight sinus ; and on the lower surface they terminate in the occi- 

 pital and lateral sinuses. 



Dissection. The pia mater and the vessels are now to be stripped 

 from the brain, and the origin of the cranial nerves is to be care- 

 fully defined. Over the cerebrum and pons, the pia mater can be 

 detached with tolerable ease by using two pairs of forceps ; but on 

 the cerebellum and the medulla oblongata the membrane adheres so 

 closely as to require much care in removing it without tearing the 

 brain- substance, or injuring the nerves. 



In clearing out the groove between the halves of the cerebellum 



