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Show 1904.] OF TUE POTTO AND SLOW LORIS. 161 varius*, which seems to resemble the two species of that genus examined by myself in the early union of the anterior cerebrals. In the first place, there is some variability in the length of the basilar artery; I mean, of course, in proportion to the size of the brain. N o w the bx-ains of the Potto, the Lemur, and Midas rosalia are approximately of the same size, the Potto being if anything the smallest of the sex-ies. The basilar artery measures in these types in the order given 13 mm., 7 mm., 6 m m . The discrepancy is too great to allow of a possible error in measurement at all sex-ious. The increase in length is naturally of the artery alone, as is shown by taking as fixed points the origins of the third and sixth nerves. I find that tlxe length of the basilar ax-tery in Lemur is proportionately equivalent to that of the same artery in Mctcacus nemestrinus, Cercocebus collaris and C. fuliginosus, Mycetes seniculus, as well as Midas rosalia. I may further-remark that a long basilar artery is found in the Carnivora and Rodentia. The basilar artery gives off, at any rate in the Px-imates, two cerebellar arteries, of which one is quite anterior and the other-arises not far from the union of the two vex-tebrals to form the basilar. The figux-es in Gray's 'Anatomy' (10th ed.) and Q train's 'Anatomy' (8th ed.) show the middle cerebellar artery ( = "anterior inferior cerebellar artery") arising in front of the origin of the sixth nerve. I find that in all the Monkeys mentioned and in the Lemurs the artery in question arises behind the sixth nerve. The circle of Willis is not precisely identical in the two Lemurs. In the Potto the basilar artery divides into oxxly two branches at the anterior end, which continue for tlxe space of aboirt 2 m m . without dividing. It bifurcates just before reaching the third nerve into the anterior cerebellar and the posteriox- cerebral arteries. The two postex-ior cerebral ax-teries are connected by a commissure in front of the end of the basilar- artery. The posterior-communicating artery has not the exact relations to the carotid that it has in Man. In Man, the cax-otid reaches the circle of Willis just at the optic chiasma, and divides at once into three branches, viz., the anterior and middle cerebrals, and the postex-ior communicating artery. In the Potto, the carotid reaches the circle of Willis behind the origins of the anterior and middle cerebral arteries and, as it were, aloxxg tlxe course of the postex-ior communicating artery. The anterior- cerebral artery of one side closely approaches its fellow of the opposite side before they turn over the end of the brain and run along over the surface of the corpus callosum. There is, however, no anterior communicating artery at all ; but the two anterior- cerebrals fuse into one on the upper surface of the corpus callosum to separate again later. The arrangement in fact is precisely that of Macacus nemestrinus. In Lemur macaco thex-e are diflerences. The basilar artery gives off on either side a slender anterior cerebellar artery before * " Zur vergleichenden Anatomie der Kopfarterieu bei den Mammalia," Dsnischr. k. Akad. Wien, lxvii. 1899, p. 677. PROC. ZOOL. Soc-1904, VOL. I. No. X L 11 |