CEREBRAL ARTERIES. 363 



II. ANTERIOR AND MIDDLE CEREBRAL ARTERIES. 



The terminal branches of the internal carotid artery supply the anterior 

 and greater part of the pia mater and brain. 



The anterior cerebral, commencing at the subdivision of the internal 

 carotid at the inner end of the fissure of Sylvius, turns forwards towards 

 the middle line to reach the longitudinal fissure between the anterior lobes 

 of the cerebral hemispheres, and is connected with the vessel of the opposite 

 side by the anterior communicating artery t a branch not more than two 

 lines in length. The two anterior cerebral arteries, lying close together, in 

 the next place turn round the anterior border of the corpus callosum, run 

 backwards on its upper surface, concealed by the cerebral hemispheres, and 

 end by anastomosing with the posterior cerebral arteries. In their course 

 they give numerous branches to the brain. 



The middle cerebral artery, larger than the anterior, inclines obliquely 

 outwards, taking the course of the fissure of Sylvius ; within this it divides 

 into several branches, which ramify in the pia mater investing the surfaces 

 of the anterior and middle lobes of the brain, and join with the branches of 

 both the anterior and posterior cerebral arteries. Numerous small branches, 

 without ramifying in the pia mater, turn upwards at once, and enter the 

 brain at the anterior perforated spot, through which they reach the corpus 

 striatum. 



One or two choroid arteries, which sometimes arise directly from the 

 internal carotid, passing backwards, enter the fissure between the middle 

 lobe and the crus cerebri, to reach the descending cornu of the lateral 

 ventricle, in which they are distributed to the choroid plexus. 



PECULIARITIES. In rare instances, the anterior cerebral arteries have united into a 

 single trunk, like the basilar artery behind, and have again divided into a right and 

 left artery. The anterior communicating artery is frequently double. 



. CIRCLE OF WILLIS. 



A remarkable anastomosis exists between the branches of the vertebral 

 and internal carotid arteries within the cranium, by which the circulation in 

 the brain may be equalised, and any irregularity which might arise from the 

 obliteration of one, or even two of the vessels, may speedily be remedied by 

 a corresponding enlargement of the others. This anastomosis, known as the 

 circle of Willis, results from a series of communications between the follow- 

 ing branches. The anterior cerebral arteries are connected together, as 

 already mentioned, in the longitudinal fissure by the anterior communicating 

 artery. The right and left internal carotids, the trunks from which the 

 anterior cerebral arteries arise, are united by the posterior communicating 

 arteries to the posterior cerebral arteries, which arise behind from a single 

 trunk the basilar artery. Within or opposite to the area of this vascular 

 circle are the following parts of the encephalon, viz., the commissure of 

 the optic nerves, lamina cinerea, infundibulum and tuber cinereum, corpora 

 albicantia, posterior perforated spot with part of the crura cerebri, and the 

 origin of the third pair of nerves. 



Frequently the posterior cerebral artery of one side arises by an enlarged posterior 

 communicating artery from the internal carotid, and is connected only by a slender 

 vessel with the basilar. 



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