412 



THE ARTERIES OF THE HEAD AND NECK. 



and left arteries (4 per cent.). The two arteries frequently differ in size at their origin, and 

 the larger one reinforces the other through the anterior communicating artery : this may 

 proceed so far that both anterior cerebral arteries spring from one internal carotid, by means 

 of a common trunk which bifurcates as it enters the longitudinal fissure. The two arteries 

 have also been seen united in a single trunk, which runs in the longitudinal fissure, giving 

 off branches to both hemispheres. On the other hand, a third artery, middle anterior cerebral, 

 arising from the anterior communicating is not uncommon (4 '5 per cent.). The anterior com- 

 municating is sometimes double (7 per cent.). 



The posterior communicating artery is sometimes connected with the middle cerebral 

 artery instead of the trunk of the internal carotid. The posterior cerebral artery frequently 

 arises by an enlarged posterior communicating artery from the internal carotid, and is con- 

 nected only by a slender vessel with the basilar (on the right side only in 5 '5, on the left side 

 only in 4'5, and on both sides in 2 per cent.). The opposite condition, in which the middle 

 cerebral artery is derived from the posterior, has been recorded by Hyrtl. The posterior 

 communicating artery is sometimes wanting, or the branch representing it fails to join the 

 posterior cerebral (on the right side only in 4*5, on the left side only in 6'o, and on both sides 



Fig. 346. THE DISTRIBUTION OF THE 



INTERNAL CAROTID AND VERTEBRAL 

 ARTERIES AT THE BASE OP THE 



BRAIN. (Allen Thomson.) ^ 



On the left side a portion of the tem- 

 poral lobe of the cerebrum has been re- 

 moved, so as to open up the fissure of 

 Sylvius and expose the island of Reil ; 

 and the left half of the cerebellum has 

 been removed to show the lower surface 

 of the posterior part of the cerebral 

 hemisphere. 1, placed on the optic 

 commissure, points to the divided left 

 internal carotid artery ; 2, left anterior 

 cerebral artery, exposed fully by the re- 

 moval of the optic nerve ; 2', placed on 

 the genu of tLe corpus callosum between 

 the two anterior cerebral arteries ; x , 

 placed on the lamina cinerea in front of 

 the optic commissure, marks the anterior 

 communicating artery; 3, left middle 

 cerebral artery, passing into the fissure 

 of Sylvius and distributing its branches 

 over the convolutions of the island of 

 Reil and others beyond ; 4, placed be- 

 tween the infundibulum and the cor- 

 pora albicantia, points by a line to the 

 left posterior communicating artery ; 5, 

 basilar artery ; 6, left posterior cerebral 

 artery ; 7, placed on the pens Varolii, 

 points to the right superior cerebellar artery ; 8, anterior cerebellar artery ; between 7 and 8, one of 

 the largest of the transverse branches of the basilar artery ; 9, 9, right and left vertebral arteries ; 

 10, inferior cerebellar artery ; 11, anterior spinal arteries. 



in 1'5 per cent.). The anterior choroid artery is occasionally derived from the middle cerebral 

 or posterior communicating : it may be represented by two or three smaller branches. (B. C. A. 

 Windle, " On the Arteries forming the Circle of Willis," Journ. Anat., xxii, 1888. A 

 determination of the frequency of the different variations in 200 cases, of which 119 were 

 normal). 



Circle of Willis. A remarkable anastomosis exists between the branches of 

 the vertebral and internal carotid arteries within the cranium, by which the circula- 

 tion is equalised in different parts of the brain, and any deficiency which might arise 

 from the obliteration of one, or even two of the vessels, may be remedied by a cor- 

 responding increase in the supply through the others. This anastomosis, known as 

 the circle of Willis, is formed in the following manner. The anterior cerebral arteries 

 are connected together, as already mentioned, by the anterior communicating artery. 

 The right and left internal carotids, the trunks from which the anterior cerebral 



