Bones of the arm

What are the bones of the arm?

In everyday speech the arm is everything from the shoulder to the hand. Anatomists split it into three parts: the shoulder girdle, the arm proper and the forearm. Together they contain five bones on each side.

The hand adds 27 more bones, which have their own section of this site. This page covers the five bones above the wrist and the joints that link them.

How are the arm bones arranged?

The bones are arranged as a chain that starts at the breastbone and ends at the wrist. The clavicle reaches out from the top of the chest to the tip of the shoulder. The scapula hangs from its outer end and lies flat on the back of the rib cage.

The humerus hangs from the scapula and runs down to the elbow. Below the elbow the chain becomes two bones. In the model the palm faces forward, so the radius is on the outer side with the thumb and the ulna is on the inner side with the little finger.

What is the shoulder girdle?

The shoulder girdle, or pectoral girdle, is the clavicle and the scapula working as one unit. The clavicle, or collarbone, is a strut that holds the shoulder out to the side. The scapula, or shoulder blade, is a flat triangle that carries the socket for the arm.

The girdle has only one bony link to the trunk, at the inner end of the clavicle. The scapula is otherwise held on the rib cage by muscles alone. This is why the shoulder blade can slide and tilt so freely.

Which bone is in the upper arm?

The upper arm has one bone, the humerus. Its rounded head faces inward and sits against the socket of the scapula. Its lower end is wide and flat, with two shaped surfaces that fit the two forearm bones.

The humerus is the longest and largest bone of the upper limb. Most of the muscles that move the shoulder and the elbow attach somewhere along it.

Which bones are in the forearm?

The forearm has two bones, the radius and the ulna. The ulna is wide at the elbow and narrow at the wrist. The radius is the opposite, narrow at the elbow and wide at the wrist.

The two bones are not fixed to each other. The radius can swing around the ulna, and this is the movement that turns the palm up or down.

What are the main joints of the arm?

The shoulder joint is a ball and socket between the head of the humerus and the shallow glenoid cavity of the scapula. The socket is small compared with the ball, which gives the shoulder the widest range of movement of any joint in the body.

The elbow is mainly a hinge. The ulna grips the humerus and bends and straightens on it, while the head of the radius rests against the humerus beside it. The elbow also houses the upper joint between the radius and the ulna.

Two smaller joints sit at the ends of the clavicle. One joins it to the breastbone and the other joins it to the acromion, the bony tip of the shoulder.

How do the arm bones connect to the hand?

The wrist joint lies between the wide lower end of the radius and the first row of carpal bones, mainly the scaphoid and the lunate. The ulna reaches the wrist but is separated from the carpal bones by a disc of cartilage, which the model does not show.

Because of this, most of the force from a push or a fall on the hand passes into the radius first. Beyond the eight carpal bones come the five metacarpals of the palm and the fourteen phalanges of the fingers and thumb.

Common problems with the arm bones

A fall on an outstretched hand sends force up the whole chain. The usual sites of a break are the lower end of the radius, the middle of the clavicle and the upper end of the humerus.

The shoulder is the joint of the body that dislocates most often, a price paid for its shallow socket. This page describes anatomy only.

About this free 3D model of the bones of the arm

The interactive 3D model on this page is free to use in your browser, with no sign-up. It is built from scan-based anatomy data, so every bone is a separate, correctly placed shape you can turn and inspect from any side.