The Mercury line, also called the health line or hepatica, runs from the base of the palm toward the Mount of Mercury below the little finger, and the tradition reads it for health, digestion and, in the older texts, for the state of the liver, which is what the name hepatica records. Its absence is generally read as a good sign, indicating that health is not a preoccupation, which is the kind of interpretation that cannot fail either way and is a fair example of why the system resists testing. It must be said explicitly that palmistry offers no diagnostic information about health, that no line on the hand indicates a disease, and that anyone with a health concern should consult a physician rather than a palmist.
Cheiro's own system gave the line a specific and rather alarming reading: he held that the Line of Health rises from the Mount of Mercury and, where it crosses toward the life line, marks the point at which an illness already developing would come to its crisis, a piece of interpretation that reads the palm as though it foretold specific medical events. Later and more cautious writers dropped that reading, and some modern schools instead treat the line as indicating skill in business and communication rather than physical health, a further sign that its meaning has never been fixed across the tradition. The claim of medical foresight is the clearest instance in the whole art of the gap between what palmistry asserts and what it can support, and it is why the line's older name, the health line, should be treated as a historical curiosity rather than as advice.
Sources Les Mysteres de la MainAdolphe Desbarrolles with Palmistry for AllCheiro (William John Warner)