61 



brales, which are secondary formations from the anastomoses of the 

 first segmental arteries. A glance at figure two will show therefore 

 the developmental origin of the a. communicans posterior and the 

 composite nature of the a. cerebri posterior. 



De Vriese (23) has also ably worked out the ontogeny of the 

 a. basilaris. She shows that the a. basilaris is, in its oldest form, 

 doubled and is composed of the two caudal branches of the internal 

 carotids. A more advanced stage is represented by the doubled a. 

 basilaris being united by transverse anastomoses, then comes the reti- 

 culated a. basilaris through an increase and complication of the trans- 



A. communicans anterior 



A. cerebri anterior 



A. cerebri media 



A. carotis interna 



A. communicans posterior 



A. cerebri posterior 



A. basilaris 

 A. vertebralis 



A. cerebralis anter. 



A. cerebralis media 



A. cranial terminal 

 branch. 



B. caudal terminal 

 branch. 



Terminal branch of A. basilaris 

 A. basilaris 



A. vertebralis 



Fig. 2. Scheme of the morphological formation of the circulus arteriosus (Willisi). 

 Slightly modified from de Vkiese. On the right is the primitive morphological con- 

 dition with DE Vriese's terminology. On the left is the adult condition as described 

 in text books of anatomy with the B.N. A. terminology. 



verse anastomoses, leading up to an unpaired a. basilaris through 

 fusion of the caudal branches of the internal carotids with definite 

 stages in amphibia, reptiles, birds, kangaroo, perissodactyla, artiodactyla, 

 and cetacea. Next comes the unpaired a. basilaris due to the fusion 

 cranially of the two terminal branches of the carotids and caudally of 

 the two aa. vertebrales; with progressive development the carotids 

 furnish less, and the vertebrals more blood with definite conditions in 

 monotremes, pinnipedia, and the majority of Carnivora. Lastly the 

 a. basilaris is a strong single vessel formed throughout by fusion of 

 the aa. vertebrales as in the opossum, edentates, rodents, insectivora, 

 cheiroptera, some Carnivora, prosimiae, simiae and man. 



