(radial deviation) and flex the hand. The APL insertion on the trapezium and the APB origin on the same bone is the only connection between the thumb's intrinsic and extrinsic muscles. As the thumb is brought into action, these two muscles must coordinate to keep the trapezium stable in the carpus, which is important for the proper functioning of the thumb (i.e. precision and power grip.) In other animals The only primates to have an APL completely separated from the extensor pollicis brevis are modern humans and gibbons. In gibbons, however, the APL originates proximally on the radius and ulna, whereas it originates in the middle part of these bones in crab-eating monkeys, bonobos, and humans. In all these primates, the muscle is inserted onto the base of the first metacarpal and sometimes onto the trapezium (siamangs and bonobos) and thumb sesamoids (crab-eating monkeys). In chimpanzees, the APL flexes the thumb rather than extends it like in modern humans. Compared to the wrists of chimpanzees, the human wrist is derived (compared to the Pan-Homo LCA) in having considerably longer muscle moment arms for a range of hand muscles. It is possible that these differences are due to the supinated position of the trapezium in humans which, in its turn, is a result of the expansion of the trapezoid on the side of the palm. A small, lens-shaped radial sesamoid embedded into the APL tendon is a primitive state found in all known Carnivora genera except in the red and giant pandas and the extinct Simocyon where it is hypertrophied (enlarged) into a sixth digit or a so-called "false thumb", a derived trait that first appeared in ursids. The APL sesamoid is present in all non-human primates, but only in about half of gorillas, and normally absent in humans. Gallery See also Extensor pollicis longus De Quervain syndrome Bennett's fracture Notes References Sources Muscles of the upper limb