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Midbrain vascular casts of the Lylei's flying foxes (Pteropus lylei)Â were prepared by infusion of Batson's No.17 plastic mixture into the blood vessels and examined by stereomicroscopy and scanning electron microscopy. Histological study of the midbrain was also performed. It was found that the midbrain of Lylei's flying fox was supplied by the branches of the vertebrobasilar system. These branches gave off the penetrating arteries, which coursed radially into the internal part of the midbrain to ward the cerebral aqueduct. These arteries, which could be divided into anteromedial, anterolateral, posterolateral and posteromedial groups, according to the points of entry and supplying areas. The arteries ramified into arterioles and capillaries, respectively. The density of the capillary network in the midbrain was closely related to the density of the nerve cells in midbrain nuclei. Less vascularity was found in the areas occupied by nerve fibers. The arterial anastomoses could be observed on the surface of the midbrain. The venous drainage in the midbrain could be divided into three major groups according to the areas of drainage. Firstly, anterior or petrosal group drained the blood from the areas ventral to cerebral aqueduct into the superior petrosal sinus. Secondly, the superior or galenic group emptied the venous blood from the thalamocollicular and dorsal aqueductal veins into the great cerebral vein of Galen and rectus sinus, respectively. Thirdly, the posterior group collected blood from the collicular veins into the rectus sinus. Finally, both rectus and superior petrosal sinuses drained into the external jugular vein and partially into the internal jugular vein.
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