STUDIES OF THE HANDS OF LIVING PRIMATES

TLDR

The anatomical basis of the passive movements and the double‑locking mechanism of the anthropoid ape hand is discussed. The study examined passive movements and prehensile patterns in two young chimpanzees and one young orangutan, all about two years old. Passive movements of the ape hand resemble those of a human child, except for limited wrist extension and MCP joint constraints, and a double‑locking mechanism at the 2–5th digits was identified; active prehension shows a functional duality but differs from humans, likely due to thumb‑finger length disproportion, with the double‑locking mechanism compensating for thumb ineffectiveness.

Abstract

A study of passive movements and an analysis of the prehensile patterns of movement of the anthropoid ape hand was carried out in two young chimpanzees and one young orangutan, all approximately two years old at the time of study. The passive movements compare closely with those of a human child except in respect of the wrist and metacarpophalangeal joints of the fingers; in the case of the former joint, while flexion was very full, extension was limited. Passive movement of the metacarpo‐phalangeal joints was also very full, movement being limited only by contact of the fingers with the palm. A “double‐locking” mechanism of the 2–5th digits taking place at the metacarpo‐phalangeal joints is described. The anatomical basis of these movements and the “double‐locking” mechanism is discussed. Active movements were studied in terms of the prehensile patterns of the human hand (Napier, 1956) and it is shown that, while there is evidence of a functional duality of prehension in the hands of young anthropoid apes, the execution of the two grips differs profoundly from that of Man; the difference is discussed in terms of the disproportion in the length of the thumb and fingers in apes. It is suggested that the “double‐locking” mechanism of the fingers compensates in some measure for the ineffectiveness of the thumb in prehension.