Walk Like a Sloth–lesson 2: the ulna

Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

compassGetting Oriented

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.

sloth ulna hi


ulna and radius


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