The arm consists of the long humerus bone and the forearm bones, the radius and the ulna. The connection between the humerus and forearm bones forms the elbow joint. The metacarpal bones form the wrist, the carpals form the hand, and the phalanges form the fingers and thumbs. Important bony projections on the humerus serve as locations for muscle and ligament attachments and can also be the sites for injury. When the arm is in the anatomical position (palms up), the medial epicondyle is the rounded part of the bone on the inside of the elbow and the lateral epicondyle is the one on the outside. These two sites are attachment points for the medial and lateral collateral ligaments that hold the humerus to the radius and ulna. They are also the points where many wrist and finger flexor and extensor muscles originate. Since many of these muscles attach at the same site, their tendons (which attach muscle to bone) are often referred to in groups: the common extensor tendons (CET) and the common flexor tendons (CFT).
