1905.] ARTERIES OF THE BRAIN IN BIRDS. 117 



as a rule, but not always, in front of the sixth nerves. The 

 position of these arteries is nearly exactly that of the middle 

 cerebellar arteries in Mammalia. Each cerebellar artery emits 

 a considei'able branch to the dorsal side of the spinal cord before 

 passing, as it almost always (if not quite always) appears to do, 

 over the flocculus. The cerebellum is also supplied with blood 

 by a branch of the ai'teiy supplying the corpoi-a bigemina and by a 

 branch of the posteiior cei'ebral artery. 



The Basilar artery is short in extent, and is as a rule connected 

 with only one carotid, either right or left, though sometimes with 

 both. This asymmetrical condition of the basilar artery is very 

 rarely to be seen in Mammals, but it is cpiite characteristic of 

 Birds. 



The following synopsis shows the connection of the basilar 

 artery with the circle of Willis in the types examined : — 



Basilar artery bifurcate anteriorly, completing circle of Willis 

 posteriorly. 



a. Branches equal or subequal : Syrnium, Asio. 



h. Left-hand branch larger : Struthio, Cathartes, Goura. 



c. Right-hand branch larger : Droviceus^ Ara, Tantalus. 



Basilar artery connected with right side only of circle of Willis : 

 Aquila, Psoiyhia, Anthropoides, Ca7'iama, Gymiiorhina. 



Basilar artery connected with left side oidy of circle of Willis : 

 Falco, Pelecanus, Sjjheuiscas. 



I am unwilling to comment at length upon these facts since 

 they are but few. They allow, however, of some obvious reflections 

 and some generalisations which can hardly be reversed by subse- 

 quent discovery. On the assumption (which seems to be reason- 

 able) that the complete and equal division of the basilar artery 

 anteriorly to join both carotids i^ primitive, it seems certain that 

 the production of an asymmetry due to the shrinking or complete 

 suppression of one branch has proceeded along several distinct 

 lines, quite irrespective of such relationships as other anatomical 

 characters allow us to surmise. That Aquila and Falco fall into 

 different categories is sufficient proof of this. On the other hand, 

 the fact must not be lost sight of that the Crane-like birds 

 (viz., Psophia, Cariama, scnA Anthropoides) have apparently followed 

 the same path of modification. 



It is furthermore interesting to note that the Striges are unlike 

 the Falcones, and that Cathartes is unlike either. 



