DISTRIBUTION OF THE CEREBRAL ARTERIES. 413 



arteries arise, are united by the posterior communicating arteries to the posterior 

 cerebral arteries, and these 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 optic commissure, infundibulum and tuber cinereum, corpora 

 albicantia, and posterior perforated spot. 



DISTRIBUTION OF THE CEREBRAL ARTERIES. 



From the three cerebral arteries anterior, middle and posterior" two distinct 

 systems of vessels proceed. The first of these, consisting of branches given off by 

 the arteries while still within or immediately after leaving the circle of Willis, is 

 destined for the great central ganglia, and is called the central system of arteries. 

 The vessels of the second set ramify in the pia mater, and are distributed to the 

 grey matter of the convolutions and the subjacent white matter : they constitute 

 the cortical system of arteries. These two systems are independent of each other, and 

 the vessels of the central system are all " terminal arteries," that is, do not form 

 anastomoses together ; but the cortical arteries communicate more or less freely on 

 the surface of the hemisphere through their secondary and tertiary ramifications. 

 The cortical arteries break up into fine branches in the pia mater before entering 

 the cerebral substance. 



The anterior cerebral artery has a very limited central distribution, giving only 

 a few small branches (and these not constantly) which constitute the antero-mesial 

 group of central arteries. They pierce the inner part of the anterior perforated 

 space, and supply the anterior extremity of the caudate nucleus. Two or three 

 twigs, which are sometimes included in this group, commonly pass from the anterior 

 communicating artery to the lamina cinerea, the rostrum of the corpus callosum and 

 the adjacent part of the septum lucidum. Its cortical branches are (a) two to four 

 small inferior internal frontal arteries to the internal orbital convolution, supplying 

 also the olfactory lobe ; (&) the anterior internal frontal artery, distributed to the 

 lower part of the marginal convolution and the superior and middle frontal convo- 

 lutions ; (e) the middle internal frontal artery to the callosal convolution and upper 

 part of the marginal convolution ; and (d) the posterior internal frontal artery, sup- 

 plying the quadrate lobule and giving off the artery of the corpus callosum, which 

 runs backwards on the upper surface of that body to its posterior extremity. 



The middle cerebral artery immediately after leaving the internal carotid 

 gives off the more numerous antero-lateral central arteries, which pass directly up- 

 wards through the foramina of the anterior perforated space to the base of the corpus 

 striatum. They are divided by Duret into lenticular arteries supplying the internal 

 and middle segments of the lenticular nucleus and the internal capsule, lenticulo- 

 striate arteries to the outer segment of the lenticular nucleus, external capsule and 

 caudate nucleus, and lenticulo-optic arteries to the outer and posterior part of the 

 lenticular nucleus and the outer part of the optic thalamus. One of the lenticulo- 

 striate arteries, larger than the rest, is especially noteworthy as being a frequent seat 

 of cerebral haemorrhage : it passes outwards and upwards at the junction of the 

 anterior and middle thirds of the lenticular nucleus, between the latter and the 

 external capsule, and perforates the internal capsule to end in the caudate nucleus. 

 The main trunk of the middle cerebral passes upwards and outwards in the fissure of 

 Sylvius to the surface of the island of Eeil, where it divides into the following four 

 branches : (a) the inferior external frontal is limited in its distribution to the outer 

 part of the orbital surface of the hemisphere and the adjacent inferior frontal 

 convolution ; (b) the ascending frontal is distributed to the convolution of the same 

 name, together with the root of the middle frontal convolution ; (c) the ascending 



