The middle finger is the finger of Saturn, named for the Mount of Saturn beneath it, and the naming carries the planet's whole character with it. Saturn in astrology signifies limit, weight, time, duty and the things that do not come easily, and the tradition reads the middle finger for exactly those: seriousness, caution, prudence and a sense of responsibility, turning in the unfavourable reading into gloom, suspicion and a narrow spirit. A finger leaning toward Jupiter is read as ambition tempered; one leaning toward Apollo, as gravity lightened.
The Saturnian vocabulary spreads further here than at any other digit. The fate line, which runs up the middle of the palm toward this finger, is called the line of Saturn for no reason except where it is headed, and Cheiro treats the two together under one heading. That is the tell of a borrowed system: the line is named after the mount, the mount after the planet, the finger after the mount, and at no point in the chain does anything about the hand enter.
It is the longest digit, which the tradition takes as fitting for the element it says governs the rest, and it is also the digit whose relative length varies least from hand to hand, so the feature carrying the heaviest reading is the one with the least to distinguish it. The reading is a traditional interpretive practice, and it has not been shown to correspond to anything about the person whose hand it is.
Sources Introductiones apotelesmaticae elegantes in chyromantiam, physiognomiam, astrologiam naturalem, complexiones hominum, naturas planetarumJohannes ab Indagine (Johannes Rosenbach) with Palmistry for AllCheiro (William John Warner)