1905. 1 ARTERIES OF THE BRAIN IN BIRDS. 109 



been able to see. The middle cerebral (Sylvian) ai'tery curves 

 round on each side towards the middle line, but not with so 

 marked a flexure as in the Owls. 



The brain of Falco lanarius differs in some few particulars from 

 that of Aquila, but agrees in other points. The cerebellar arteries 

 are not asymmetrical, but the basilar is, though its asymmetry is 

 different. It communicates, in fact, chiefly if not entirely with 

 the left carotid instead of the right. The main branch of the 

 Sylvian shows the same flexure as that of Aquila. 



PelecamiiS fuscus (text-fig. 17, p. 108). — One marked feature of 

 this brain is the absolutely unpaired character of the anteidor spinal 

 artery, in the course of which I could detect no bifurcation and 

 reunion. The cerebellar arteries arise behind the 6th nerve and 

 ai'e slightly asymmetrical, the right being in advance of the left. 

 The main peculiarity of these artei-ies is the fact that on the right 

 side the posterior spinal artery does not arise as a branch of the 

 cerebellar, but as a separate trunk from the basilar artery. On 

 the left side the artery is not thus independent, but arises very 

 early from the cerebellar. The main trunk of the basilar is con- 

 tinued into the left side of the circle of Willis. The circle termi- 

 nates on either side anteriorly in an unusual way. The ophthalmic 

 arteries (text-fig. 17, Opth.) do not, as is the rule with birds, arise 

 in front of the optic chiasma and form practically the anterior 

 termination of the circle of Willis. They resemble those of 

 mammals, in arising hetioeen the origins of the posterior and middle 

 cerebral (Sylvian) arteries. The anterior cerebral artery divides 

 on each side into two branches. On the left side the second, 

 innermost, branch is the main trunk, and passes along the inter- 

 hemispheral groove to the olfactory bulbs, which are very little 

 marked. 



Spheniscios demersus (text -fig. 18, p. 110). — In this brain the an- 

 terior spinal and the basilar arteries are nearly perfectly continuous, 

 and the former is in no place double, as it so frequently is in other 

 birds. The junction of the anterior spinal artery with the basilar 

 is efiected through the right cerebellar artery, an asymmetry which 

 is common in the avian brain. The basilar artery has a much 

 greater calibre than the ensuing anterior spinal, quite three 

 times as great. The anterior spinal also contrasts, by its 

 slenderness, with the two stout cerebellar arteiies, each of which 

 is considerably more than half the diameter of the parent basilar 

 artery. The cerebellai' arteries arise from the basilar artery 

 behind the point of origin of the sixth pair of cranial nerves. 

 Anteriorly the basilar ai'tery bends to the left and becomes con- 

 tinuous with the carotid ; there is no trace, that I could discover, 

 of a bifurcation and a branch to the right carotid. From each 

 half of the incompleted circle of Willis four principal arteries and 

 one of minor importance arise, before the circle ends anteriorly in 



