1905.] ARTERIAL SYSTEM IX 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 off branches to the cerebrum and also to the 

 cerebellum. 



(6) In front of this artery and also in front of the carotid is an 

 arteiy 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 o/Testudo vicina. 



The most salient characteristic of the ai-tei-ial 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 

 lai-ger 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 cai-otid in front of the origin of the 

 third nerve. Behind the origin of the third nerve a large number 

 of ai-teries 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 jy«r 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 lai'ger artery : it partly supplies the cerebral hemi- 

 spheres 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 ai'teries 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 i-unning 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 



5* 



