The anterior inferior cerebellar artery; its variations, pontine distribution, and significance in the surgery of cerebello-pontine angle tumours.

The anterior inferior cerebellar artery arises constantly from the basilar artery on each side at the junction of its lowest and middle thirds and passes laterally and downwards to cross the cerebello-pontine angle and the eighth cranial nerve. At this point the internal auditory artery is usually given off, passing almost immediately into the internal auditory meatus. Either immediately before or iimediately after crossing the eighth nerve, the main artery divides into two. (1) One branch passes laterally and downwards on the medial and anterior border of the cerebellar hemisphere. After about a centimetre or so of tortuous course it sends a constant branch of very variable siize posteriorly along the medial surface of the hemisphere to anastomose with a cerebellar branch of the posterior inferior cerebellar artery. Reference will be made to this anastomosis later. (2) The other branch of the anterior inferior cerebellar artery passes directly laterally and curls round the upper edge of the flocculus, where it lies on the surface of the middle cerebellar peduncle, and it then passes on to the cerebellar hemisphere proper and anastomoses with all three main cerebellar arteries. This latter branch of the main artery which passes immediately laterally gives off small arteries which' supply the middle cerebellar peduncle and adjoining part of the pons. The main artery itself also sends small vessels into the pons, supplying chiefly its lateral part as far superiorly as the junction of the upper and middle thirds and extending down to supply the lateral third of the upper part of the medulla oblongata. Plate Ha is taken from a specimen of the basilar arterial system which has been injected post-mortem with red latex. The anterior inferior cerebellar artery on each side will be seen as described,'and most significant is that part of the artery which crosses the cerebello-pontine angle, for here it lies