Correct answer: In my source this was actually referred to as ectopic mammary gland tissue. It is unclear to me when it's called polymastia or ectopic tissue, since a nipple doesn't have to be formed in order to have it called a breast and apparantly it can be called ectopic tissue even when a nipple is present. I do know the milk line extends from the arm down to the legs, but I don't know how far exactly.