116 MU. F. E. BEDBARD ON THE [Feb. 7, 



deep down in the cleft between tlie corpus bigeminum and the 

 cerebrum. In Gymnorhina the conditions are rather different. 

 The artery in question at least mainly supplies the corpus 

 bigeminum of its side, and can be seen I'unning over the anterior 

 face of that division of the brain. Another ixnusual chai-acteristic 

 shown in the brain is the origin from a common stem of both the 

 middle and anterior cerebral arteries. The latter, as will be seen 

 from the drawing, are particularly small. The middle cerebral 

 arteries have thi'ee main branches ; but it is possible that the 

 innermost bi-anch is to be looked upon as the trvie anterior cerebral 

 artery, since its distribution agrees very closely with a separately 

 arising artery in many birds (see text-figs. 16, 18, A.cer.^ pp. 106 

 & 110), which I have ventured to call anterior cerebral. 



§ General Account of the Cerebral Arterial System in Ai^es. 



From the details set forth in the preceding descriptions of 

 various types of birds' brains, it is possible to extract a geneiTtl 

 accoimt of the artei^ies as characterising birds. 



The circle of Willis is never fully complete ; it is invariably in- 

 complete anteriorly, there being no anterior communicating artery 

 as in mammals ; posteriorly the asymmetrical disposition of the 

 basilar artery usually (but not always) fails to bring abovit a 

 direct union between the two carotids. 



The t%oo Carotids are invariably both present and are posterior 

 in position, never entering the imperfect circle of Willis towards 

 the middle of its course. They are also perfectly symmetrical 

 Avith each other and equisized. They lie behind the last cerebral 

 artery, and not, as in mammals, between the mitldle and the 

 posterior cerebral artery. They are alone concerned wdth the 

 circulation in the brain, the vertebral arteries being unimportant. 



The Ophthalmic arteries are always large and symmetrical, and 

 their position varies slightly, arising as they do either behind 

 the origin of the middle and anterior cerebrals or in common with 

 the latter. In the former case, the point of origin of the 

 ophthalmic arteries resembles that of the Mammalia. In the 

 latter, which is the more usual, the condition is typical of birds 

 as opposed to mammals. They are never small and inconspicuolis 

 as is the case in some mammals. 



There are invariably three pairs of Cerebral arteries, of wdiich 

 the anterior is distinctly less important than the middle and the 

 posterior, and its supply of blood to the hemispheres is limited to 

 the anterior and inferior regions of the hemispheres and to the 

 rudimentary olfactory lobes. The cerebral ai'teries are the only 

 arteries snpplj^ing the brain wdiich arise from the circle of Willis. 

 The intercerebral region is not supplied, as in mammals, by the 

 anterior cerebral artery, but by the middle and posterior. 



There is one pi-incipal Cerebellar artery on each side, arising 

 from the basilar artery where the latter becomes continuous wdth 

 the anterior spinal artery nt about the middle of the medulla, and 



