The lines a chiromant reads are flexion creases, and they have an anatomy of their own that is independent of anything read from them. The primary creases form in the womb, around the twelfth and thirteenth weeks, as the hand begins to fold; they mark where the skin is tethered to the fascia beneath so that it hinges cleanly instead of bunching. They are set before birth and change very little afterward, which is the first thing a reader should know about them.
What the tradition calls the Mercury Line is a secondary crease running toward the little finger. It is often faint or missing, and unlike the primaries it can alter with hand use over a lifetime, which is a confound for any system that reads it as fixed.
It is worth separating chiromancy from dermatoglyphics, which is often confused with it and is a different subject. Dermatoglyphics studies the ridge patterns of the skin, the loops and whorls used in fingerprinting, and it has a genuine research literature including work on developmental correlates. Palmistry reads the flexion creases, which are a different structure entirely, and there is no body of evidence connecting crease form to character, career or lifespan. The atlas records what the chiromantic tradition says about this line because that tradition is a real object of study. It does not endorse the reading.