Introduction to Walk Like a Sloth: lessons in ground sloth locomotion
The lower arms of mammals have two bones–the ulna, which is located on the little-finger side of the arm, and the radius which sits on the thumb side. The narrow end of the ulna (called the head) is the distal or wrist end; the wider end with the notch is the proximal or elbow end. The notch is called the greater sigmoid cavity or semilunar notch (literally “half-moon”), it rotates on the humerus letting you move your forearm up and down. The projecting bone above and behind the notch is the elbow or olecranon. A small depression below the semilunar notch and to the side is called the lesser sigmoid cavity or radial notch, where the radius articulates with the ulna. A rough bump on the wrist end marks the anchoring point for a tendon that connects the two bones in sloths.