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.
We have already observed that the external carotid is sometimes deficient, in which case, the internal is a continuation of the common carotid, and gives off the branches which usually arise from the external ; and that it sometimes arose near the base of the skull. In many cases we find that at the side of the sella turcica it passes through a foramen formed by the existence of a middle clinoid process, or spicula of bone connecting the tip of the anterior clinoid process to the side of the body of the sphenoid bone. In some cases its anterior branch to the cerebrum unites directly with that of the opposite side, instead of being connected with it by one or more transverse branches, and, after a short course, the common trunk divides into two branches. Occasionally its posterior communicating branch is of considerable size, and its continuation forms the posterior artery of the cerebrum, being connected to the basilar trunk by a short branch. There are cases on record, in which Haller had observed the two anterior arteries of the cerebrum furnished by the carotid of one side, and the two middle arteries of the cerebrum furnished by the carotid of the opposite side.
* Green, p. 10. † Op. Cit., p. 9.
‡ Cruveilhier, Ang., p. 102. § Meyer, Lehre von der Blut, p. 49.
|| Green, p. 9. ¶ Soemmering, Op. cit. p. 126.