Circle of Willis 43 



the right side. This statement, however, is not apparently borne out 

 by statistics ; probably more ' left ' cases are reported, because of the 

 interesting clinical feature aphasia being present. Acute rheuma- \ 

 tism, gout, atheroma, morbus cordis, granular disease of the kidney, ] 

 syphilis, and injury are the chief causes of disturbance of arterial cir- V 

 culation in the motor area. And he who knows himself to be the subject 

 of one or more of these conditions should try to avoid everything likely 

 to put a strain upon his arterial system, such as running to catch a J 

 train, straining at stool, and so on. 



The common seat of cerebral haemorrhage is in the neighbourhood 

 of the corpus striatum, and occurs from the good-sized branches which 

 run straight up from the beginning of the middle cerebral artery into 

 the lenticular and caudate nuclei ; probably it is the directness of the 

 course of these branches from the main trunk which causes them to 

 burst under the shock of the ventricular contraction. 



The posterior communicating artery passes from the back of the 

 internal carotid to join the posterior cerebral. Just there, also, the 

 carotid gives off the antetior choroid twigs, which, entering the de- 

 scending cornu of the lateral ventricle, supply the hippocampus and the 

 choroid plexus. 



The posterior cerebrals come off at the bifurcation of the basilar, 

 and wind round the crura to supply the occipital lobes, anastomosing 

 there with the middle and anterior cerebrals. The posterior cerebral 

 is joined by the posterior communicating from the internal carotid ; it 

 gives offsets to the optic thalamus, which enter by the posterior per- 

 forated space, and \hzposteiior choroid twigs, which pass beneath the 

 corpus callosum to the velum interpositum. 



The circle of Willis is an arrangement for equalising the flow of 

 blood between the internal carotid and the basilar, and between these 

 trunks on the two sides of the middle line. Except for this arrange- 

 ment, ligature of the common carotid would probably be followed by 

 rapid degeneration of the brain. The vessels forming the circle are 

 the anterior communicating, anterior cerebral, internal carotid, posterior 

 communicating, posterior cerebral, and basilar. The circular arrange- 

 ment does not always suffice for carrying on the supply across the 

 middle line, for sometimes, as a direct result of ligation of the common 

 carotid, apoplexy or softening occurs. The walls of the cerebral 

 arteries are so thin that these vessels look like veins ; they inosculate 

 very freely in the pia mater, but their terminal branches do not ana- 

 stomose. This last fact accounts for the complete loss of function of 

 a part when its artery becomes plugged. 



Within the circle are the lamina cinerea, optic commissure, in- 

 fundibulum and tuber cinereum, corpora albicantia, and posterior 

 perforated space. 



The cerebellum derives its supply from the posterior inferior cere- 

 bellar of the vertebral, and from the anterior inferior and the superior 



