This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The blood reaches the brain by means of the two internal carotid and the two vertebral arteries. The vertebrals enter through the foramen magnum and unite to form the basilar, which at the upper border of the pons divides into the two posterior cerebrals. These give off two small branches which go to the internal carotids; they are the posterior communicating arteries. The carotids divide into the anterior and middle cerebral arteries, the anterior communicating with one another by means of the anterior communicating artery. Thus we have the circle of Willis (circulus arteriosus), formed by the posterior cerebral, posterior communicating, internal carotid, anterior cerebral, and anterior communicating arteries on each side.
Fig. 29. - Circulus arteriosus or circle of Willis.
The blood supply of the brain is divided into an anterior division, furnished by the carotids, and a posterior division, supplied through the basilar and posterior cerebrals. The communication branch running between these two sets of vessels is so small that if either is occluded the supply of blood is practically cut off from that point and ischaemia results, at least in most cases.
The anterior division is subdivided into a right and a left half by the two carotid arteries. These communicate across the median line through the anterior cerebral and anterior communicating. Here again the anterior communicating branch is so small that it is sometimes unable to furnish blood to the opposite side of the brain when the carotid supply of one side is shut off. This may cause syncope, stupor or delirium, hemiplegia, and often softening and death. These results have not infrequently followed ligation of the carotid artery in cases of aneurism. Obstruction of one vertebral artery would produce no effect because circulation would be restored by the other vertebral through the basilar.
Fig. 30. - The internal carotid artery in its course through the skull, showing its relations to the jugular vein and cranial nerves. The Gasserian ganglion has been raised from its bed and turned forward.
Fig. 31. - Distribution of the A. cerebri anterior and A. cerebri posterior on the medial surface of the brain.
The internal carotid artery (Fig. 30) enters the petrous portion of the temporal bone, then turns inward and upward through the foramen lacerum medium, then forward through the cavernous sinus and finally turning
A. cerebri anterior upward gives off the ophthalmic artery; it then pierces the dura mater just behind the anterior clinoid process, where, after giving off the posterior communicating and anterior choroid, it divides into the anterior and middle cerebral arteries.
Fig. 32. - Distribution of the A. cerebri anterior, A. cerebri posterior, and A. cerebri media on the lateral surface of the brain.
Fig. 33. - Arteries of the base of the brain, especially the branches of the middle cerebral giving rise to apoplexy.