120 THE NERVOUS SYSTEM 



central branches, owing to the upward course which these 

 arteries take. 



The Anterior Cerebral Artery, at its origin from the internal 

 carotid, at once bends abruptly in a medial direction to gain 

 the posterior extremity of the longitudinal fissure. It then 

 bends sharply forwards and is continued round the genu of 

 the corpus callosum in company with the artery of the oppo- 

 site side. Together they extend backwards at the bottom of 

 the longitudinal fissure until they reach the parietal lobes. 



The anterior cerebral artery is responsible for the supply 

 of the whole of the medial aspect of the hemisphere as 

 far back as the precuneus, and, in addition, its cortical 

 branches emerge from the longitudinal fissure to supply the 

 cortex of the lateral and orbital aspects near their margins. 

 The anterior cerebral artery, therefore, supplies the upper 

 extremity of the precentral gyms, which contains the motor 

 centres for the lower limb. 



The Vertebral Artery, which arises from the first part of 

 the subclavian in the root of the neck, passes upwards, 

 traversing the foramina in the transverse processes of the 

 cervical vertebrae, and enters the skull through the foramen 

 magnum. Within the skull, it ascends on the lateral aspect 

 of the medulla oblongata and inclines medially to meet its 

 fellow of the opposite side in the median plane at the lower 

 border of the pons. At this point the two vertebral arteries 

 unite to form the Basilar Artery, which passes upwards in 

 the median plane to the upper border of the pons, where it 

 divides into the two posterior cerebral arteries. 



The Posterior Cerebral Artery runs laterally and backwards 

 round the mid-brain, to which it supplies central branches. 

 Its cortical branches are distributed to the posterior two- 

 thirds of the inferior surface of the cerebral hemisphere, 

 and, in addition, they supply the cortex of the whole of the 

 occipital lobe. The posterior cerebral artery is, therefore, 

 responsible for the blood-supply of the higher visual centres, 

 and, on this account, an embolus which finds its way into 



