Humerus

The humerus is a long bone in the arm that runs from the shoulder to the elbow. It helps us move our arms. The humerus connects to other bones, such as the scapula, radius, and ulna.

This bone has three main sections. The top part, called the upper extremity, has a rounded head, a narrow neck, and two small bumps known as tubercles. In the middle, the shaft of the humerus is mostly round.

At the lower end, called the lower extremity, there are two small rounded bumps named epicondyles, and two other bumps called the trochlea and capitulum.

Etymology

The word "humerus" comes from a Latin word meaning "upper arm" or "shoulder". It is related to words in other languages, like a Gothic word for "shoulder" and a Greek word for "shoulder".

Structure

The upper part of the humerus has a large rounded head joined to the body by a narrow neck. There are also two bumps called the greater and lesser tubercles.

Head

The head (caput humeri) is almost round. It points upward and inward, and a little backward. It connects with the glenoid cavity of the scapula to form the glenohumeral joint (shoulder joint).

Anatomical neck

The anatomical neck (collum anatomicum) is slanting and forms an obtuse angle with the body.

Surgical neck

The surgical neck is a narrow area below the tubercles. It is a common place for a fracture. It touches the axillary nerve and the posterior humeral circumflex artery.

Greater tubercle

The greater tubercle (tuberculum majus; greater tuberosity) is a large bump placed toward the back and to the side.

Lesser tubercle

The lesser tubercle (tuberculum minus; lesser tuberosity) is smaller and placed toward the front and side of the head of the humerus.

Bicipital groove

The tubercles are separated by a deep groove, the bicipital groove (intertubercular groove; bicipital sulcus), which holds the long tendon of the biceps brachii muscle.

Shaft

The body or shaft of the humerus is triangular to cylindrical in cross section. It has 3 surfaces.

Distal humerus

The lower part of the humerus is flattened from front to back, and curved slightly forward. It ends in a broad surface divided into two parts by a small ridge. On either side are the lateral and medial epicondyles.

Articular surface

The articular surface goes a little lower than the epicondyles, and is curved slightly forward.

Fossae

Above the front of the trochlea is a small dip, the coronoid fossa, which holds the coronoid process of the ulna when the forearm bends.

Above the back of the trochlea is a deep triangular dip, the olecranon fossa, which holds the top of the olecranon when the forearm straightens.

Epicondyles

The epicondyles connect above with the supracondylar ridges.

• The lateral epicondyle is a small, bumpy point, curved a little forward.
• The medial epicondyle, larger and more noticeable than the lateral, points a little backward.

Articulations

At the shoulder, the head of the humerus connects with the glenoid fossa of the scapula. At the elbow, the capitulum of the humerus connects with the head of the radius, and the trochlea of the humerus connects with the trochlear notch of the ulna.

Nerves

The axillary nerve is found at the top end, near the shoulder.

The radial nerve runs close to the humerus.

The ulnar nerve lies at the lower end of the humerus near the elbow.

Function

Muscular attachment

The deltoid muscle starts on the clavicle, acromion, and the spine of the scapula. It attaches to the deltoid tuberosity of the humerus and helps lift, straighten, and move the shoulder in a circle. The supraspinatus also starts on the spine of the scapula and attaches to the greater tubercle of the humerus, helping to lift the shoulder.

The pectoralis major, teres major, and latissimus dorsi attach to the intertubercular groove of the humerus. They help bring the arm toward the body and turn it inward. The infraspinatus and teres minor attach to the greater tubercle and help turn the arm outward. The subscapularis muscle attaches to the lesser tubercle and helps turn the arm inward.

The biceps brachii, brachialis, and brachioradialis bend the elbow. The triceps brachii and anconeus straighten the elbow and attach to the back side of the humerus.

The supraspinatus, infraspinatus, teres minor, and subscapularis muscles form a group called the rotator cuff. This group helps keep the shoulder joint stable. The other muscles balance movements like lifting and pressing.

Other animals

Early amphibians had short limbs because they did not have a strong shaft. In most land animals, the humerus looks similar to that in humans. Some reptiles and a few mammals have a hole in the lower part of their humerus called the entepicondylar foramen. This helps nerves and blood vessels pass through.

Additional images

Here are some images showing different views and parts of the humerus bone and related joints:

• Position of humerus (shown in red). Animation.
• Left humerus. Animation.
• 3D image
• Human arm bones diagram.
• Humerus - inferior epiphysis. Anterior view.
• Trochlea. Posterior view.
• Humerus - inferior epiphysis. Posterior view.
• Humerus - superior epiphysis. Anterior view.
• Humerus - superior epiphysis. Posterior view.
• Elbow joint. Deep dissection. Anterior view.
• Elbow joint. Deep dissection. Posterior view.
• Elbow joint. Deep dissection. Posterior view.
• The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula
• Left humerus. Anterior view.
• Left humerus. Posterior view.
• Left humerus. Anteriolateral view.
• Left humerus. Medial view.
• Fracture of the proximal humerus
• Left elbow-joint, showing anterior and ulnar collateral ligaments.
• Capsule of elbow-joint (distended). Posterior aspect.
• Humerus anatomy

Ossification

Further information: ossification

The humerus is one of the first bones to form when a baby is growing in the womb. It starts with a hard spot in the middle of the bone. At birth, only the middle part of the humerus is hard, while the ends are still soft. Hard spots appear at the ends of the humerus around 4 months and 10 months after birth, and they slowly join together by age 3. The bone fully hardens by age 13, but keeps growing until around age 17.