Appendicular Skeleton

Anatomy of the Arm and Forearm

The arm has one large bone, the humerus, that connects to the forearm, which is made up of the radius and ulna.

The arm and forearm are made up of three long bones. The humerus is the largest bone of the upper limb and is located between the shoulder and elbow. It is the only bone located in the upper arm region. Proximally (closer to the core of the body), the humerus articulates with the scapula at the shoulder. The humerus terminates at the distal end (farther away from the core of the body) where it articulates with the bones of the forearm to form the elbow joint.

At the proximal end of the humerus is the head, which is smooth and round. This head articulates with the glenoid cavity of the scapula. Laterally (the outside region away from the midline of the body) there are two different tubercles (small, round bony projections)—the greater and lesser tubercles. Both are recognized as the sites of attachment for muscles that originate from the scapula. They also help anchor the humerus to the scapula. The anatomical neck surrounds the head. At the base of the head and tubercles is the surgical neck. The surgical neck is the start of the long shaft region of the humerus. It is named for it being a common fracture point on the bone.

Extending down from the tubercles on the lateral side of the shaft is the deltoid tuberosity (round bony projection). This is where the deltoid (shoulder) muscle attaches to the humerus. Nerves run along the radial groove that is found on the posterior side of the shaft. At the distal end of the humerus there is a series of condyles that connect the humerus to the forearm bones. Condyles are rounded bony projections at the end of a bone and are the site of joint formation. The medial epicondyle is a condyle on the lateral side of the distal end of the humerus that serves as an attachment point for the forearm muscles to the humerus. The lateral epicondyle is located on the medial side of the distal end of the humerus and also serves as an attachment point for forearm muscles. Between the medial and lateral epicondyles is a spool-shaped condyle called the trochlea. A depression immediately above the trochlea on the posterior side is called the olecranon fossa. The trochlea and olecranon fossa are important for the joint formation between the humerus and the forearm.

Each forearm includes two bones, the ulna and radius. The ulna is found on the medial side, and the radius is located on the lateral or thumb side. They extend parallel to one another down the forearm. The ulna is a long and slender bone that forms the elbow joint with the humerus. At the ulna's proximal end there is a trochlear notch that forms a tight encasing over the humerus where it articulates with the trochlea, creating the elbow joint. There are two processes (bony outgrowths) around the trochlear notch. The posterior pointy region of the elbow is the olecranon process. This process is located proximal to the trochlea of the humerus. The coronoid process is anterior to the trochlea and is distal to the olecranon process. The trochlear notch of the ulna articulates with the trochlea of the humerus. When the forearm extends (or the arm straightens), the olecranon process inserts into the olecranon process of the humerus. Moving distally, the ulna becomes narrower and forms a head at the end. On the medial side of the head is a projection called the styloid process, which is a site for ligament attachment.

The radius is a long and slender bone that is found in the forearm lateral to the ulna. The radius becomes wider at its distal end when compared to the ulna and does not attach firmly to the humerus. At its proximal end, the radial head is shaped like a wheel. It functions to rotate against the ulna and humerus as the forearm is moved. Distal to the head is a bony projection called the radial tuberosity, where the biceps brachii muscles attach. The biceps brachii muscles act to flex the elbow. The radius has a styloid process found laterally at its distal end. This serves as an attachment site for ligaments to connect the forearm to the wrist bones. The radius articulates with some bones of the wrist.
The upper arm contains one bone-the humerus-and the forearm contains two bones-the radius and ulna. The humerus and the ulna form the elbow joint.