The Mount of Venus is the large pad at the base of the thumb, bounded by the life line, and it is the most prominent feature of the palm; the tradition claims for it vitality, warmth, affection, sensuality and the capacity for pleasure, reading a full and firm mount for a generous nature and a flat one for coldness. Anatomically it is the thenar eminence, the muscle group that moves the thumb, and its bulk is chiefly a function of hand use and musculature, which is a straightforward and testable explanation for variation that the tradition does not consider. This is a general problem with the scheme: several of the features it reads as character have obvious physical causes, occupational, genetic or age-related, and the interpretive tradition was constructed without reference to them.
Traditional chiromancy reads a well-developed Mount of Venus, at the base of the thumb and encircled by the life line, as indicating vitality, warmth, sensuality, love of family and physical energy generally, drawn directly from Venus's own classical association with love and desire; it is typically read as the single most consistently significant mount in the whole scheme, given how much of the palm's base it occupies. An excessively developed mount is read as self-indulgence or an excess of physical appetite, while a flat, underdeveloped mount suggests coldness or low vitality. Because Venus mount also sits directly beneath the thumb, some manuals additionally read its texture and colour, rather than only its size, as bearing on a person's constitutional health within the tradition's own framework.
Sources Les Mysteres de la MainAdolphe Desbarrolles with Kaplan's Functional and Surgical Anatomy of the HandEmanuel B. Kaplan, 3rd edition revised by Morton Spinner