This index page points towards individual sections — see Vertebral Artery, Basilar Artery, PICA, AICA, SCA. While I think that combinining vert and basilar into one entity is a better anatomical and functional approach, more practial issues – too many pictures on one page for optimal browsing — are also important, as individual sections grow.
It is critical to realize that vertebral and basilar arteries, and their branches, should not be conceptually separated from each other. I urge an interested reader to go through all sections for complete appreciation.
For an excellent review on the subject, read an article by F. Siclari, I.M. Burger, J.H.D. Fasel, and P. Gailloud. Developmental Anatomy of the Distal Vertebral Artery in Relationship to Variants of the Posterior and Lateral Spinal Arterial Systems. AJNR Am J Neuroradiol. 2007; 28(6):1185-90 (ISSN: 0195-6108). Link to full text/PDF: http://www.ajnr.org/content/28/6/1185.full