The anterior cerebral artery ascending segment consists of A2 and A3, and its horizontal segment consists of A4 and A5.
A2 (vertical): from the anterior communicating artery, extending anterior to the lamina terminalis and along the rostrum of the corpus callosum, terminating either at the genu of the corpus callosum or at the origin of the callosomarginal artery
AC: anterior clinoid process; ICA: internal carotid artery; LT: lamina terminalis; ON: optic nerve; OlN; olfactory nerve; SW: sphenoid wing; TS: tuberculum sellae; A1: A1 segment of the Anterior Cerebral Artery; A2: A2 segment of the Anterior Cerebral Artery; M1: M1 segment of the Middle Cerebral Artery.
Picture from http://3dneuroanatomy.com
see Proximal pericallosal artery aneurysm or A2 anterior cerebral artery aneurysm