378 AETERIES OF THE HEAD AXD NECK. 



carotid artery supply the anterior and greater part of the pia mater and 

 brain. 



The anterior cerelral, commencing at the subdivision of the in- 

 ternal 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 hemisphere, and is connected with the 

 vessel of the opposite side by the anterior comimmicafing arierij, 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 cerehi'al arteries. In their course they give numerous 

 branches to the olfactory lobes and optic nerves, the under surface of 

 the anterior lobes, the third ventricles, corpus callosum and inner 

 surface of the hemisphere. 



The tnithUe cerebral artery, larger than the anterior, inclines ob- 

 liquely outwards, taking the course of tlie fissure of Sylvius ; within 

 this it divides into several branches, which ramify in the jna 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 per- 

 forated 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 ai'e distributad to the choroid plexus. 



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

 single tiTink. like the liasilar artery behind, and have again divided into a right 

 and left arteiy. The anterior communicating artery is frequently double. 



Frequently tlie posterior cerebral artery of one side arises by an enlarged pos- 

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

 a slender vessel with the basilar. 



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 en- 

 largement of the others. This anastomosis, known as the circle of 

 Willis, results from a series of communications between the following 

 brandies. The anterior cerebral arteries are connected together, as 

 already mentioned, in the longitudinal fissure by the anterior commu- 

 nicating artery. The right and left internal carotids, the trunks 

 from which the anterior cerebral arteries arise, are united by the pos- 

 terior communicating arteries to the i)osterior 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 albican tia, posterior per- 

 forated spot with part of the crura cerebri, and the origin of the third 

 pair of nerves. 



Distribution of the Arteries in the Cerebrum. — The recent researches 

 of Duret and Heubner have thrown some additional light on the circulation in 



