MEMBRANES AND BLOOD VESSELS 



cerebral springs) with the posterior cerebral. The arterial 

 ring is completed posteriorly by the bifurcation of the basilar 

 artery into the two posterior cerebral vessels (Fig. 181). As 

 a rule the circulus arteriosus is not symmetrical. One posterior 

 communicating artery is almost invariably larger than its fellow 

 of the opposite side. 



Dissection. The brain being placed with its base uppermost, the 

 dissector should proceed to remove the blood vessels and membranes from 



Anterior cerebral artery 



Anterior communicating 



Internal carotid 



Middle cerebral 



Antero-lateral basal arteries 

 Posterior communicating 



Postero-median basal arteries 

 Posterior cerebral artery 



Superior cerebellar artery 



"t 



FIG. 181. Diagram of the Circulus Arteriosus. 



its surface. This must be done with the forceps and a pair of scissors. 

 It is a dissection which requires very delicate manipulation, because the 

 cerebral nerves, at their points of attachment to the brain, are so intimately 

 connected with the pia mater that any undue traction applied to the 

 membranes will tear the nerves away. Indeed, in the case of the 

 medulla oblongata, the dissector is advised to leave the pia mater in 

 position until the nerve roots have been studied. The relation of the pia 

 mater to the fourth ventricle also renders this desirable. 



In removing the arachnoid and pia mater from the lateral surface of 

 the cerebrum, it is well to raise it first from the margins of the hemisphere, 

 and then to work towards the lateral fissure (Sylvian). By this method, the 

 membranes and vessels within this great fissure and in relation to the 

 insula can be withdrawn without damage to the brain substance. Of 

 course, at the present stage, the membranes cannot be removed from 

 every part of the brain ; but as the dissection proceeds, opportunities for 

 completing the process will arise. 

 II 29 a 



