The stuff online, 'brachiocephalic artery rising from the arch of the aorta, dividing to the carotid...' seemed to make it work. It was sometimes referred to as 'the unnamed artery'.
anonyma in British
(əˈnɒnɪmə)
noun
1. obsolete
a woman of ill repute and low social stature
2. anatomy
a main artery coming off the aorta before the left-hand carotid artery