. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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 uni
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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 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 opo
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