This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Middle Cerebral Artery is larger than the preceding, and from its size might be considered the continued trunk of the internal carotid; it sinks into the fissure of Sylvius, taking a direction outwards and backwards. It first gives a great number of branches to the inferior part of the brain, to the pia mater covering the crura cerebri, and one or more choroid branches which accompany the choroid plexus into the inferior cornu of the lateral ventricle. It then divides in the fissure of Sylvius into two considerable branches for the anterior and middle lobes of the brain; these follow the fissure outwards and backwards, and terminate near the posterior part of the brain by numerous subdivisions: some tortuous twigs are given off which sink into the anfrac-tuosities and supply the pia mater; others appear to perforate and surround the roots of the olfactory nerve.
The student should now impress on his memory the various important parts with which the internal carotid artery is connected, and the manner in which it may be affected either by disease or accident, in consequence of its vicinity to them. Thus, its relation to the tonsil points out the danger of directing the knife too deeply backwards or outwards in opening abscesses of that gland. Beclard relates a case in which an itinerant quack destroyed a patient's life in this way. The vicinity of this vessel to the organ of hearing explains the various derangements of the functions of the latter arising in consequence of an undue determination of blood to the head, and, in certain cases, the hemorrhage from the ear which occurs in consequence of fractures extending to the base of the skull.
I am not aware that there is any case on record of aneurism of the trunk of the internal carotid, though its branches are frequently the seat of this disease. In one case, however, in which Sir A. Cooper operated successfully, he was of opinion that the disease was in this vessel, and not in the external carotid.*
Near the base of the skull the internal carotid artery in graminivorous animals divides into several minute branches, which form a plexus of vessels called the rete mirabile of Galen ; these subsequently unite into a single trunk, which afterwards divides into its cerebral branches. The use of this peculiar plexiform arrangement is to prevent the brain from being injured by the gravitation of the blood whilst the animal is grazing. A similar arrangement of the ophthalmic artery, " rete ophthalmi-cum," has been observed at the back of the orbit in birds.