776 THE VASCULAR SYSTEM. 7 
the chorioidal fissure, and, after entering the posterior part of the velum interpositum, end 
in the chorioid plexus in the body of the lateral ventricle and the upper part of its 
descending cornu. 
(B) Cortical.—(b!) The anterior temporal, frequently a single branch of variable size, 
is not uncommonly replaced by several small branches. It supplies the anterior parts of 
the uncinate and the occipito-temporal convolutions. 
(02) The posterior temporal is a larger branch than the anterior. It supplies the 
posterior part of the uncinate gyrus, the greater part of the occipito-temporal convolution, 
and the lingual lobule. 
(4°) The calcarine branch is the continuation of the posterior cerebral artery along 
the calcarine fissure. It supplies the cuneate lobe, the lingual lobule, and the posterior 
part of the outer surface of the occipital lobe. 
(o*) The parieto-occipital branch, smaller than the calcarine, passes along the 
corresponding fissure to the cuneus and precuneus. 
Circle of Willis.—The cerebral arteries of opposite sides are intimately con- 
nected together at the base of the brain by anastomosing channels. Thus the 
two anterior cerebral arteries are connected with one another by the anterior 
communicating artery, whilst the two posterior cerebrals are in continuity through 
the basilar artery from which they rise. There is also a free anastomosis on each 
side between the carotid system of cerebral arteries and the vertebral system by 
means of the posterior communicating arteries, which connect the internal carotid 
trunks and posterior cerebral arteries. 
The vessels referred to form the so-called circle of Willis (circulus arteriosus 
[Willisi]). This is situated at the base of the brain, in the region of the inter- 
peduncular space, and encloses the following structures: the posterior perforated 
space, the corpora albicantia, the tuber cinereum, the infundibulum, and 
the optic commissure. The “circle” is irregularly polygonal in outline, and 1s 
formed posteriorly by the termination of the basilar and by the two posterior 
cerebral arteries, postero-laterally by the posterior communicating arteries and the 
internal carotids, antero-laterally by the anterior cerebral arteries, and in front by 
the anterior communicating artery. 
It is stated that this free anastomosis equalises the flow of blood to the various 
parts of the cerebrum, and provides for the continuation of a regular blood-supply 
if one or more of the main trunks of the basal vessels should be obstructed. 
ARTERIES OF THE UPPER EXTREMITY. 
The main arterial stem of each upper extremity passes through the root of the 
neck, traverses the axillary space, and is continued through the upper arm to the 
forearm. In the latter it only runs a short distance, terminating just below the 
bend of the elbow by bifureating into the radial and ulnar arteries which descend 
through the forearm to the hand. That portion of the common trunk which lies 
in the root of the neck is known as the subclavian artery, the part in the axillary 
space is termed the axillary artery, whilst the remaining part is called the brachial 
artery. 
THE SUBCLAVIAN ARTERIES. 
On the right side the subclavian artery (a. subclavia, Figs. 552 and 554) com- 
mences at the termination of the innominate artery behind the sterno-clavicular 
articulation, whilst that on the left side arises from the arch of the aorta behind 
the lower part of the manubrium sterni. 
The right artery is about three inches long (75 mm.), and it lies in the root of 
the neck. The left artery is about four inches (100 mm.) long, and is situated not 
only in the root of the neck, but also in the superior mediastinal part of the thorax. 
In the root of the neck each artery arches outwards across the apex of the lung and 
behind the anterior scalene muscle, and is divided into three parts, which le 
respectively to the inner side, behind, and to the outer side of the muscle. The 
extent to which the arch rises above the level of the clavicle varies considerably, 
and not uncommonly it reaches the level of the lower part of the thyroid body. 
