DISTEIBUTIOX OF VESSELS IX THE BKAIX. 379 



the brain. According to these authors, the three great arteries which go to the 

 cerebrum — the anterior, middle, and posterior cerebral — give origin to two very- 

 distinct systems of vessels. The fii"st of these, consisting of branches given 

 off by the arteries immediately after they leave the circle of "Willis, is destined 

 to the great central ganglia, and is called the central sy.stcni of ariericx ; the 

 other ramifies in the pia mater, and is distributed to the gi'ey matter of the con- 

 volutions and the subjacent white matter ; it is called the cortical xiixtrm of 

 arteries. These two systems are to a great extent independent of each other, 

 the vessels which connect them being few in number, and of almost capillary 

 minuteness. Moreover, not only are the two systems thus distinct, but tho 

 branches of the several arteries are also limited in their distribution to certain 

 well-defined areas, and their communicat'ions are few and small, and occur only 

 at the periphery of their respective areas of distribution, so as to render the areas 

 practically independent territories. The same remark applies, in less degree, 

 however, to the secondary, and even tertiary di\'ision of these arteries. 



The anterior cerebral lias a very limited central distribution, giving only a few 

 small branches (and these liable to inucli variation as to size and number) to the 

 anterior extremity of the corpus striatum. Its cortical branches are three in 

 number : the first being distributed to the two internal orbital convolutions ; 

 the second to the anterior extreinity of the marginal convolution, to the superior, 

 and to the anterior jiortion of the middle frontal convolutions on the outer sur- 

 face ; the third to the inner surface of the liemisphere as far as the extremity of 

 the calloso-marginal fissure ; the fourth to the quadrate lobule ; and from this 

 last branch the arter}- of the coi-pus callosum is given off. 



Tho middle cerebral immediately after leaving the internal carotid gives off a 

 number of small vessels which pass directly upwards, parallel to each other, into 

 the foramina of the anterior perforated space, and enter the base of the corpus 

 striatum. They are distributed to the two extraventricular nuclei (grey nucleus 

 and lenticular nucleus) of that body, to the posterior part of the nucleus candatus 

 and the portion which borders upon the optic thalamus. The main trunk of the 

 middle cerebral passes upwards and outwards in the fissure of Sylvius until it 

 reaches the island of Reil, on the surface of which it divides into four branches. 

 The first branch is limited in its distribution to the outer jjart of the orbital 

 surface and the adjacent inferior frontal convolution : the second branch sup- 

 plies the posterior part of the middle frontal, and the chief part of the ascending 

 frontal convolution ; the third branch passes in the fissure of Rolando to the 

 rest of the ascending frontal and to the ascending parietal convolution and to 

 the anterior part of the supei-ior parietal lobule ; and the fourth, h'ing in the 

 posterior branch of the fissure of Sj-h'ius, sui^plies the inferior parietal lobule, 

 and the superior temporo-sphenoidal convolution. 



TlxQ jmsterior cerebral axtexies, {\he origin and course of which are described 

 at p. 386) gives ofE a number of twigs in the posterior jDerforated spot, and 

 others as it passes round the eras, both of which sets pass into the thalami optici. 

 crura cerebri and corpora quadrigemina. The cortical branches are three in 

 number : the first is distributed to the anterior part of the uncinate gyi'us and 

 its immediate vicinitj' ; the second branch supplies the jiosterior part of the 

 uncinate gyrus and the lower part of the temporo-sphenoidal lobe ; the third, 

 lying in the calcarine fissure, supplies the occipital lobe on its inner and outer 

 surfaces. (H. Buret, in Archives de Physiol. 1S74 ; Heubner, in Centralblatt fiir 

 die Med. Wissensch. 1872.) 



SUBCLAVIAN AETERIES. 



The subclavian artery is only the commencing portion of a long 

 trunk which forms tlie main artery of the upper limb, and which i.s 

 artificially divided for purposes of description into three parts, named 

 the subclavian, axillary, and brachial arteries. 



The subclavian artery, arising on the right side from the extremity 

 of the innominate stem, and on the left from the arch of the aorta, 

 passes a short way up into the neck, arches outwards over the pleura. 



