Joseph Sheppard - Anatomy: A Complete Guide for Artists

The shoulder socket is made up of two bones: the clavicle (collarbone) in front and the scapula (shoulder bone) in back. The upper arm has one bone: the humerus. Half its dual head is shaped like a rectangle and half like a ball, fitting into the shoulder socket. At the elbow, the humerus is divided into three equal sections: the first section is shaped like a ball; the second section is shaped like a spool and forms an angle downward toward the torso; the third section protrudes and can be seen and felt on the inside of the elbow. The forearm has two bones: the radius and the ulna. The radius is on the outside of the elbow and on the thumb side of the wrist. It is smaller at the elbow and larger at the wrist. The head of the radius is round and flat and rotates on the small ball of the humerus. The head of the ulna rises up behind the humerus and rides on the spool of the humerus.

There is a slight drop, or stepdown, where the clavicle meets the scapula. The muscle that attaches from the scapula to the inside of the humerus is called the coracobrachialis. It raises the arm. The small muscle attached to the inside of the humerus at the elbow is a pronator and turns the radius so that the palm of the hand faces in. The other three muscles are flexors of the wrist and fingers.

The muscle attached to the lower front of the humerus and the ulna is called the brachialis. It flexes the forearm. The long supinator attaches to the outside of the humerus and along the edge of the radius near the thumb. It pulls the radius to the supinated position (palm out). The long supinator and the pronator of the radius turn the forearm in opposite directions (see next drawing).

The largest muscle on the front of the arm is the biceps. It has two heads that attach to the scapula. At its base, the biceps also splits in two. The primary function of the biceps is to flex the forearm. It also helps rotate the radius outward.

Attached to the external third of the clavicle and to the scapula is the deltoid. It overlaps the biceps and coracobrachialis and inserts into the outside of the humerus. The deltoid elevates the arm and helps draw it forward and backward.

The ball of the humerus fits into the shoulder socket of the scapula. Note that the angle of the forearm away from torso is in a supinated position. This angle is caused by the angle of the spool on the end of the humerus (see detail at right). The hook of the ulna fits up into the back of the humerus. Note the angle at wrist.