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Show 1905.] ARTERIAL SYSTEM IN SAUROPSIDA. 67 far forward as compared with the Lacertilia ; but this appearance is at least partly due to the great length of the bifurcate region of the basilar artery in Python as compared with that of any Lacertilian. (5) The artery to the corpus bigeminum on each side arises behind the entrance of the carotid instead of in front as in Lacertilia. It gives oft' branches to the cerebrum and also to the cerebellum. (6) In front of this artery and also in front of the carotid is an artery which runs towards the optic chiasma, (7) There is a very marked completion of the circle of Willis anteriorly. (8) There is a strongly marked asymmetry in the arterial system of the brain due to the greater size of the left carotid. § Brain of Testudo vicina. The most salient characteristic of the arterial system in this Reptile is the double basilar artery (text-fig. 20, p. 68). The artery is double for the whole of its course beneath the medulla oblongata, The anterior spinal artery in fact divides into two well behind the medulla. The right-hand one of the two branches is not larger than the left; the two arteries do not run close side by side, but are separated by a considerable distance. They are joined each of them by the carotid in front of the origin of the third nerve. Behind the origin of the third nerve a large number of arteries arise from the basilar on each side; there are certainly eight or nine of them on each side, and they supply the cerebellum, the medulla, and the cranial nerves of this region of the brain. The fifth artery (on the right side at any rate), which arises from the basilar behind the third nerve, is par excellence the cerebellar artery ; it fuses with its fellow of the opposite side at the end of the cerebellum. In front of the third nerve arise two arteries rather close together, of which the anterior has several branches and is the larger artery : it partly supplies the cerebral hemispheres and corresponds, as I imagine, to that artery in the Lacertilia which supplies the corpus bigeminum on each side. As in the Lacertilia, there are two cerebral arteries on each side. The first and largest of these (text-fig. 21, p. 68) may be termed the Sylvian, as it runs along the lateral groove upon the hemisphere which has been compared to the Sylvian fissure of mammals The branches of this artery are not altogether symmetrical on tne two sides of the body; it is possible, however, to distinguish the main trunk which runs towards the top of the brain, where it divides into a forwardly running and a backwardly running branch, several branches from the main stem which pass backwards over the temporal region of the hemisphere, and a strong branch running forwards to the olfactory lobe. Moreover, there is plain on one side a branch arising immediately after the origin of the Sylvian artery, which plunges at once beneath the hemisphere. A second |