1904.] OF THE 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 antevioi- cerebrals. 

 In the fii'st jjlace, there is some variability in the length of the 

 basilar artery ; I mean, of course, in pi'oportion to the size of the 

 bi-ain, ISTow the brains of the Potto, the Leviiir, and Midas rosalia 

 are approximately of the same size, the Potto being if anything 

 the smallest of the series. The basilar artery measures in these 

 types in the order given 13 mm., 7 mm., 6 mm. The discrepancy 

 is too great to allow of a possible error in measurement at all 

 serious. The increase in length is naturally of the ai'tery alone, 

 as is shown by taking as fixed points the origins of the third and 

 sixth nerves. I find that the length of the basilar artery in 

 Lemur is proportionately equivalent to that of the same arteiy in 

 Macacus nemestrimts, Ceixocehus collaris and G. fuliginosus, 

 Mycetes senictolus, as well as Midas rosalia. I may further 

 remark that a long basilar krtery is found in the Oarnivora and 

 Rodentia. The basilar ai-tery gives off, at any rate in the Piimates, 

 two cerebellar ai'teries, of which one is quite anteiior and the othei- 

 arises not fai- from the union of the two vertebrals to form the 

 basilar. The figures in Gray's ' Anatomy' (10th ed.) and Quain's 

 'Anatomy ' (8th ed.) show the middle cerebellar artery ( = "anterior 

 infeiior cerebellar ai'teiy ") arising in front of the origin of the 

 sixth nerve. I find that in all the Monkeys mentioned and in the 

 Lemurs the ai'tery in question arises hehitul the sixth nei've. 



The circle of Willis is not precisely identical in the two Lemurs. 

 In the Potto the basilar artery divides into only two branches at 

 the anterior end, which continue for the space of about 2 mm. 

 without dividing. It bifurcates just before reaching the third 

 nerve into the antei'ior cerebellar and the posterior cerebral 

 ai'tei'ies. The two posterior cerebral arteries are connected by a 

 commissure in front of the end of the basilar artery. The posterior 

 communicating ai'teiy has not the exact relations to the carotid 

 that it has in Man. In Man, the carotid 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 posterior 

 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, along the course of the posterior 

 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 bi'ain and run along over the surface of the 

 corpus callosum. There is, however, no anterior communicating- 

 artery at all ; but the two anterior cei-ebrals fuse into one on the 

 upper surface of the corpus callosum to separate again latei*. The 

 arrangement in fact is precisely that of Macacus nemestrinids. 



In Lemur macaco thei'e are diSerences. The basilar artery gives 

 off on either side a slender anteiior cerebellar arteiy before 



* " Zur vergleicheiiden Anatomie der Kopfartericn bei deii Mammalia," Denkschi. 

 k. Akad. Wien, Ixvii. 1899, p. 677. 



Proc. Zool. Soc— 1904, Vol. I. No. XL 11 



