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Arteries the Cerebral Circulation

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THE CEREBRAL CIRCULATION, ARTERIES The Arterial Circle (Circulus arteriosis, Willisi).—The arteries which supply the cerebrum freely communicate in the arterial circle, which is really a heptagon extending from a point in the longitudinal fissure anterior to the optic chiasma, back to the pons (Fig. 9). It is about an inch and a half long, and from a half to one inch in transverse diameter. In front are the anterior cerebral arteries converging forward from the internal carotids and, through the anterior communicating artery (a. communicans anterior), uniting just as they enter the longitudinal fissure of the cerebrum. These vessels form three sides of the heptagon and the front of the circle. On either side, the posterior communicating artery (a. communicans posterior) which connects the internal carotid with the pos terior cerebral artery, forms the lateral boundary of the circle. The posterior cerebral arteries bound the circle behind, and so complete it (Fig. io). The large distal branches of the arteries which are connected with the arterial circle are distributed chiefly to the cortex and medulla of the hemispheres; while the small proximal branches supply the ganglia of the cerebrum. The former belong to the cortical system (Ai), the latter to the ganglionic system (A2).

Ai. The Cortical System of Arteries.—The cortical arterial system comprises the distal portions of the anterior, middle and posterior cerebral and the chorioidal arteries. The branches of these great vessels pierce the hemispheres perpendicularly to the surface. They are distributed, the short, to the cortex, and the long, to the medulla of the hemispheres. To a limited extent they anastomose with one another, but they do not communi cate with the ganglionic system.

The anterior cerebral artery (a. cerebri anterior, Figs. 9 and 12), a branch of the internal carotid, runs forward and toward the median line above the optic nerve and enters the longi tudinal fissures; it is here joined to its mate by a very short artery, the anterior communicating. Winding around the genu of the corpus callosum, it runs back on the medial surface of the hemisphere to the occipito-parietal sulcus. It gives origin to the antero-median ganglionic arteries, and to four groups of cortical branches: (I) The medial orbital artery (Figs. 9 and io) which supplies the medial orbital gyrus, the gyrus rectus, the optic chiasma and the olfactory bulb, tract, medial and intermediate strie, triangle, and parolfactory area. (2) The anterior medial

branch (Fig. 12) which enters the anterior parts of the gyrus cinguli and superior frontal gyrus on the medial surface and of the superior and middle frontal gyri on the convex surface.

(3) The intermediate medial branches, which are distributed to the middle part of the gyrus cinguli, to the paracentral lobule and to the upper portions of the superior frontal and the anterior and posterior central gyri. (4) The posterior medial branches, which run back to the occipito-parietal sulcus. They supply nearly the whole corpus callosum, the posterior half of the gyrus cinguli, a part of the paracentral lobule, the pracuneus, and the superior parietal lobule.

The middle cerebral artery (a. cerebri media, Figs. io and I I) crosses the anterior perforated spot and runs in the lateral fissure of the cerebrum to the posterior sulcus circularis (Reili) where it breaks up into several parieto-temporal branches. It gives origin to the antero-lateral ganglionic arteries, and to four cortical branches : (I) The lateral orbital branches are dis tributed to the anterior, lateral, and posterior orbital and the inferior frontal gyri. (2) The ascending frontal, two branches, which follow the precentral and central sulci, supply the anterior central gyrus and the posterior fourth of the middle frontal gyrus. (3) The ascending parietal, whose course is along the interparietal sulcus, furnishes blood to the posterior central gyrus and the adjacent parts of the superior and inferior parietal lobules. (4) The parieto-temporal arteries, which comprise two polar branches to the temporal lobe and a large posterior branch. The latter runs in the posterior ramus of the lateral cerebral fissure to its upturned posterior end and there bifurcates into a parietal and a temporal branch, which just pass the anterior limit of the occipital lobe. The entire distribution of the parieto-temporal arteries is to the temporal pole and to the superior, middle and part of the inferior temporal gyri; to the major parts of the supramarginal, angular and post-parietal gyri, and to a very small portion of the superior and lateral occipital gyri.

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