The anterior inferior cerebellar artery (a. cerebelli anterior inferior, Fig. 9) is given off by the basilar near the junction of its inferior and middle thirds. (Sometimes it is replaced by two or three small vessels.) It runs lateralward, behind the flocculus, keeping close to the anterior border of the hemisphere. In its course it passes anterior to the abducent nerve and posterior to the facial and auditory nerves. It supplies the anterior part of the under surface and border of the cerebellar hemisphere.
The posterior inferior cerebellar artery (a. cerebelli inferior posterior, Fig. 9) is the largest branch of the vertebral and is given off just before the vertebral arteries unite and form the basilar. Passing first between the root-bundles of the hypo glossal nerve and then between those of the accessory and vagus nerves, the posterior inferior cerebellar artery bends at a right angle backward and runs between the medulla and the cere bellar hemisphere, where it divides into a medial and a lateral branch. The medial branch follows the sulcus valleculw and gives branches to the medial part of the hemisphere and the vermis inferior. It anastomoses with its fellow of the opposite side. The lateral branch, runs lateralward from the posterior cerebellar notch over the inferior surface of the hemisphere; its terminal branches wind around the postero-lateral border and communicate with the superior cerebellar artery on the upper surface of the hemisphere. The undivided trunk of the pos
terior inferior cerebellar artery gives small branches to the medulla oblongata and supplies the chorioid tela of the fourth ventricle.
The internal cerebellar veins bring the blood from the interior of the organ and pour it into the superior and inferior external veins.
The superior external cerebellar veins (vence cerebelli supe riores) converge forward into a medial vein, which empties into the great cerebral vein, and several lateral veins, which end in the transverse or the superior petrosal sinus.
The inferior external cerebellar veins (vence cerebelli infe rores) also form one small medial vein, which runs backward and upward either into the straight or transverse sinus, and a number of lateral veins. The lateral inferior cerebellar veins terminate in the inferior petrosal and in the occipital sinus.
Lymphatics.—There are no lymphatic vessels in the cere bellum. Perivascular lymph spaces carry out the lymph from the whole brain and spinal cord and pour it chiefly into the subarachnoid space.