and the lumbricals, are active components of the finger's extensor mechanism. Fibers from some of the interossei contribute directly to the extensor hoods that wrap around the proximal phalanges, while other fibers may contribute to the central tendon and lateral bands of the mechanism. All three intrinsic groups of muscles pass palmar to the axis of the metacarpophalangeal joints, and therefore contribute to flexion there. Extension at the interphalangeal joints cannot be produced by the extensor digitorum alone, but active contraction of one of the three aforementioned intrinsic groups will because of their direct contribution to the extensor mechanism. Other animals The pollical palmar interosseous muscle (PPIM) is absent in non-human primates, and is probably an autapomorphic muscle unique to the human thumb (together with flexor pollicis longus) which probably evolved from the oblique portion of adductor pollicis. In African apes, adductor pollicis is notably well-developed, with an origin on the carpus and its ligaments, and an insertion that has migrated distally, in some cases as far as the distal phalanx. The insertion of the PPIM into the extensor mechanism is likely to have evolved with tool usage in early hominids. As comparative anatomy studies of the human PPIM strongly suggest that the muscle is evolutionarily derived from the adductor pollicis, it has been proposed that PPIM should be designated by the name musculus adductor pollicis accessorius, which indicates that the muscle is most likely a de novo structure derived from the adductor pollicis. Additional Images See also Interosseous muscles of the hand Dorsal interossei of the hand Interosseous muscles of the foot Dorsal interossei of the foot Plantar interossei muscles Notes References Muscles of the upper limb