Leo takes the heart, and this is the assignment modern readers find plausible, because Leo is ruled by the Sun, the Sun is the centre of its system, and the heart is central to the body in a way everyone accepts. The correspondence feels less arbitrary than the knees under Capricorn.
It is worth resisting that feeling, because it is exactly how a scheme of this kind earns credit it has not established. Melothesia assigns twelve signs to twelve regions in a fixed head to foot order. Whatever the heart's region turned out to be, some sign would have taken it, and any sign that took it would then have acquired a plausible reading, since every sign has enough associations attached to make a case after the fact. The apparent fit is a property of the reasoning available afterwards, not evidence about the assignment.
This matters for how the atlas reads the whole doctrine. The scheme is a real and historically important system that structured European medical practice for centuries, and its internal logic is worth understanding. It is not a set of discoveries about the body, and the assignments that happen to feel apt are not the ones with better support; they are the ones whose post hoc justification is easiest to construct.
Nothing on this page is a claim about the heart as an organ, and nothing in it is guidance.