Human and ape hands. Historical artwork of the hand of a human (left) and a barbary ape (Macaca sylvanus, right). The skin has been removed to reveal the underlying muscles and tendons. The human hand has a thick muscle that lies over the tendon to the thumb. This allows the thumb to be opposable to the other fingers. In the ape hand, the tendon to the thumb follows a similar path to those leading to the fingers, meaning it can only be moved in the same way as those digits.