1905.] ARTERIES OF THE BRAIN IN BIRDS. 107 



carotid. This arrangement contrasts with that to be observed in 

 the Penguin, where the origin of the artery of the corpus bi- 

 geminum is in front of the junction of the carotid artery with the 

 circle of Willis. 



Each artery to the corpus bigeminum divides, first of all, into 

 two principal branches, which do not, however, correspond exactly 

 with the two branches of the same artery, for example, in Spheniscus 

 (described below) ; for in Ara the anterior branch supplies both 

 the median and anterior regions of its corpus bigeminum, and the 

 general lie of the arteries is quite different, as will be seen on a 

 comparison of text-figs. 16 (p. 106) & 18 (p. 110). Both branches 

 are equisized and are symmeti'ical on the two sides of the brain. 

 They run parallel with each other for a considerable distance after 

 the origin of the main stem from the circle of Willis. The circle 

 of "Willis has a more markedly triangular shape in this Macaw 

 than in many birds ; and the transverse diameter of the triangle 

 is gi-eater than its antero-posterior diameter. A comparison of 

 text-figs. 16 (p. 106) & 20 (p. 115) will illustrate this peculiarity 

 in the circle of Willis of A7'a. 



The middle cerebral arteries are the most important of the 

 three cerebi-al arteries arising on each side. Each springs from a 

 basal angle of the tiiangular circle of Willis. At the end of the 

 " Sylvian fissure " each vessel splits into three or four trunks, of 

 which that which bends inwards towards the middle ventral line 

 cannot be said to form the main stem of the artery, any more than 

 can the others. The posterior cerebral artery arises just behind 

 the middle. The anteiior cerebral artery is a much more slender 

 artery than either of the others, and it arises further away from 

 the origin of the middle cerebral than is usual among birds, where 

 the exti-eme opposite is shown in G//mnorhina, by the common 

 origin of both anterior and middle cerebrals. 



S^rnmm aluco. — The most characteristic feature of the arteries 

 in tliis bird is the mai-ked symmetry of their arrangement, which, 

 as will have been and will be noted, is not by any means usual 

 among birds. The posterior spinal aitery is only double for a 

 shoi't distance. The cerebellar arteries aiise exactly opposite to 

 each other and in front of the 6th nerve. The two branches of 

 the basilar which form the posteiior communicating arteries are 

 perfectly equal in size. The ophthalmic arteries form the anteiior 

 termination on each side of the carotids ; they arise from the circle 

 of Willis just opposite the middle cerebral arteries, which supply 

 the whole of the fore part of the hemisphere ; there are no inde- 

 pendent anterior cerebral ai-tei'ies. The broader and shorter 

 cerebral hemispheres of Syrnmm are correlated with a somewhat 

 different biunching of the middle cerebral arteries. Each ai-tery 

 is curved in a semicircular fashion, and follows a transversely- 

 i-unning forward branch of the Sylvian fissm-e, to end in the 

 immediate neighbourhood of the olfactory lobes. Only one branch 

 of imjiortance is given ofl' from the inner side of each semicircle 



