Humerus

The humerus is a long bone in the arm that runs from the shoulder to the elbow. It is an important part of our body that helps us move our arms. The humerus connects to other bones, such as the scapula, radius, and ulna, which are all important for how our arms work.

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 but changes shape a little toward the bottom.

At the lower end, called the lower extremity, there are two small rounded bumps named epicondyles, two other bumps called the trochlea and capitulum, and three small pits known as radial fossa, coronoid fossa, and olecranon fossa. The humerus is very important for doctors, especially when it breaks near a special area called the surgical neck.

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 or proximal extremity of the humerus consists of the bone's large rounded head joined to the body by a constricted portion called the neck, and two eminences, the greater and lesser tubercles.

Head

The head (caput humeri) is nearly hemispherical in form. It is directed upward, medialward, and a little backward, and articulates with the glenoid cavity of the scapula to form the glenohumeral joint (shoulder joint).

Anatomical neck

The anatomical neck (collum anatomicum) is obliquely directed, forming an obtuse angle with the body.

Surgical neck

The surgical neck is a narrow area distal to the tubercles that is a common site of fracture. It makes contact with the axillary nerve and the posterior humeral circumflex artery.

Greater tubercle

The greater tubercle (tuberculum majus; greater tuberosity) is a large, posteriorly placed projection that is placed laterally.

Lesser tubercle

The lesser tubercle (tuberculum minus; lesser tuberosity) is smaller, anterolaterally placed to the head of the humerus.

Bicipital groove

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

Shaft

The body or shaft of the humerus is triangular to cylindrical in cut section and is compressed anteroposteriorly. It has 3 surfaces.

Distal humerus

The distal or lower extremity of the humerus is flattened from before backward, and curved slightly forward; it ends below in a broad, articular surface, which is divided into two parts by a slight ridge. Projecting on either side are the lateral and medial epicondyles.

Articular surface

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

Fossae

Above the front part of the trochlea is a small depression, the coronoid fossa, which receives the coronoid process of the ulna during flexion of the forearm.

Above the back part of the trochlea is a deep triangular depression, the olecranon fossa, in which the summit of the olecranon is received in extension of the forearm.

Epicondyles

The epicondyles are continuous above with the supracondylar ridges.

• The lateral epicondyle is a small, tuberculated eminence, curved a little forward.
• The medial epicondyle, larger and more prominent than the lateral, is directed a little backward.

Articulations

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

Nerves

The axillary nerve is located at the proximal end, against the shoulder girdle.

The radial nerve follows the humerus closely.

The ulnar nerve lies at the distal 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 fossils of amphibians had very short limbs because they lacked a strong shaft connecting the upper and lower parts. In most animals that walk on land, called tetrapods, the humerus looks much like it does in humans. Some reptiles and a few mammals have a special hole in the lower part of their humerus called the entepicondylar foramen, which helps nerves and blood vessels pass through.

Additional images

The following images show 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 during a baby's growth in the womb. It starts with a hard spot in the middle part 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.