1454 A MANUAL OF ANATOMY 



third cranial nerve. It then winds round the outer side of the 

 cms cerebri, lying between it and the hippocampal gyrus, and above 

 the fourth cranial nerve. In this manner it reaches the tentorial 

 or inferior and mesial surfaces of the occipital lobe of the cerebral 

 hemisphere. It receives, not far from its commencement, the 

 posterior communicating artery. 



Branches. — These are arranged in two groups — ganglionic and 

 cortical. 



The ganglionic group includes three sets of branches, namely, 

 postero-mesial, postero-lateral, and posterior choroidal. 



The postero-mesial ganglionic arteries pass internal to the crus 

 cerebri, and pierce the locus perforatus posticus. They supply the 

 inner part of the crus cerebri and the posterior part of the optic 

 thalamus. 



The postero-lateral ganglionic arteries pass on the outer side of the 

 crus cerebri, and supply the outer part of the crus, the posterior 

 part of the optic thalamus, the corpora quadrigemina, and the 

 corpora geniculata. 



The posterior choroidal arteries are two or three in number, and 

 pass through the choroidal fissure to the velum interpositum, which 

 they supply, together with the choroid plexus of the lateral ven- 

 tricle, and the corresponding choroid plexus of the third ventricle. 



The cortical branches are as follows : (i) anterior temporal, to 

 the anterior parts of the occipito- temporal and hippocampal gyri ; 



(2) posterior temporal, to the posterior parts of the occipito-temporal 

 and hippocampal gyri, and the inferior temporal gyrus ; and 



(3) occipital, to the occipital lobe. One of the occipital branches 

 is called the calcarine artery. It lies in the calcarine fissure, and 

 supplies the lingual or infracalcarine gyrus and the cuneus. 



Circle of Willis. — ^This circle is formed by (i) the anastomoses 

 which take place between (a) the internal carotid and posterior 

 cerebral arteries, through means of the posterior communicating 

 arteries, and (b) the anterior cerebral arteries, through means of the 

 anterior communicating artery ; and (2) the connection between the 

 two posterior cerebral arteries which is established by the basilar 

 artery. It is situated at the base of the brain, and, though spoken 

 of as a circle, it is reaJIy a heptagon — that is to say, it has seven 

 sides and seven angles. 



The circle is formed in front by the anterior communicating artery, 

 which connects the two anterior cerebral arteries ; behind, by the 

 basilar artery, as it divides into the two posterior cerebral arteries ; 

 and on either side by (i) the anterior cerebral artery, (2) the trunk 

 of the internal carotid artery, (3) the posterior communicating 

 artery, and (4) the posterior cerebral artery, in this order from 

 before backwards. The circle of Willis furnishes twigs to the grey 

 cortex of the interpeduncular region. It serves to equalize the 

 blood-pressure in the cerebral arteries, and it also provides for the 

 regular supply of blood to the brain in cases where one of the 

 main arterial trunks may be obstructed. 



