59 



possible to distinguish between several types of distribution of the 

 cerebral arteries among the different divisions of the sauropsida. In 

 the testudo the most salient characteristic of the arterial system at 

 the base of the brain is the double basilar artery, which remains 

 double for the whole of its course beneath the medulla oblongata. 

 To Beddard a completed circle of Willis appears to be a secondary 

 modification associated with strong arteries in the anterior region of 

 the brain in close communication, or rather in close apposition, and 

 there is thus apt to be confluence between closely apposed vessels and 

 spaces. If this view be correct, says Beddard, we can set aside the 

 brain of the python and that of testudo as showing primitive characters 

 by virtue of the fact that they have a closed circle of Willis, and in 

 addition to this it may be pointed out that the asymmetry of the 

 arterial system in the snake indicated by the carotids and the changes 

 in the disposition of the vessels due to the prevalence of the left 

 carotid can be fairly regarded as being secondary . . . The extra- 

 ordinary modification of the basilar artery in testudo, found nowhere 

 else, would sem, for that very reason to be a divergence from the 

 original condition. Cavatorti's case, already quoted, would thus seem 

 to be a reversion to a very primitive type. 



In the aves paper (19), Beddard's most interesting observation 

 is that the circle of Willis is never fully complete; it is invariably 

 incomplete anteriorly where there is not anterior communicating artery 

 as in mammals ; posteriorly the asymmetrieal disposition of the basilar 

 artery usually, but not always, fails to bring about a direct union 

 between the two carotids . . . 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 rarely to be seen in mammals but is quite characte- 

 ristic of birds. None of the variations recorded in this paper belong 

 to the aves type as in all the circulus arteriosus (Willisi) was always 

 completed irrespective of the basilar abnormality. 



In his mammals paper (20) Beddard comments on the much greater 

 proportionate length of the basilar artery in all the lower mammals 

 as contrasted with Man and the Primates and adds that the circle of 

 Willis is generally, but not always entirely symmetrical in itself and 

 in its branches. When incomplete or asymmetrical the advantage 

 generally lies with the left side. From his numerous observations 

 Beddard thinks that a completed circle of Wallis is a secondary 

 modification. 



From the standpoint of the abnormalities of the present paper 



