BLOOD VESSELS OF THE BRAIN 389 



The dissector should note that the branches of the middle 

 cerebral artery supply the greater part of the motor area of 

 the cortex, the greater part of the area for ordinary sensa- 

 tion, and area for hearing (cp. Figs. 148 and 153). 



Dissection. When the distribution of the branches of the 

 cerebral arteries has been noted, the dissectors must remove 

 the blood vessels and the remains of the arachnoid first from 

 the base of the brain and then from the supero -lateral surfaces 

 of the hemispheres, commencing with the base. The dissection 

 must be done with forceps and a pair of scissors. In the basal 

 region very delicate manipulation is necessary, 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. 



The removal of the arachnoid and pia mater from the supero- 

 lateral surface must be commenced at the margins of that 

 surface, and the membranes must be reflected towards the 

 lateral fissure. When the margins of the fissure are reached 

 they must be pulled apart, then the larger branches and the 

 associated parts of the membranes, which lie in the fissure, can 

 be seen, but they must not be removed at present. Cut through 

 the membranes and the vessels along the margins of the fissure, 

 but leave their deeper parts in situ. 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. 



After the vessels and the membranes are removed from the 

 base and from the supero-lateral surfaces of the hemispheres, 

 the dissectors should commence their more detailed study of 

 the brain by the investigation of the base. 



THE BASE OF THE BRAIN. 



When the membranes and the blood vessels are removed 

 from the base of the brain two large rope-like strands, 

 called the pedunculi cerebri (O.T. crura), will be seen 

 issuing from the upper part of the pons. As the peduncles 

 emerge from the pons they are close together, but they 

 diverge as they pass upwards and forwards, and, finally, 

 each peduncle disappears into the base of the corresponding 

 cerebral hemisphere. As each peduncle passes into the corre- 

 sponding hemisphere it is embraced, on its lateral side, by the 

 hippocampal gyrus, but between the gyrus and the peduncle 

 is a white, flattened band, called the optic tract^ which is closely 



in 25 a 



