The humerus is a long bone forming the skeleton of the upper arm. It extends between the shoulder and the elbow. The humerus is the longest and largest bone of the upper limb.

Like other long bones, the humerus has three main parts - a proximal end, a shaft, and a distal end.

The proximal end of the humerus articulates with the glenoid cavity of the scapula at the glenohumeral joint. At the distal end, the humerus articulates with the head of the radius and the trochlear notch of the ulna, forming the elbow joint.

Proximal end of humerus

The proximal end (or proximal extremity) of the humerus consists of the head, an anatomical neck and the greater and lesser tubercles.

The head of the humerus has a hemispherical shape and has a smooth articular surface, which is covered with hyaline cartilage. In the anatomical position, the head faces in a medial, superior and posterior direction, where it articulates with the glenoid cavity of the scapula.
The anatomical neck of the humerus is a narrowing below the articular surface of the head. The joint capsule of the shoulder joint attaches here.
The greater tubercle is the most lateral part of the proximal end of the humerus. It is marked by three impressions at the upper posterior aspect, which serve for the attachment of muscles. From superior to inferior, the muscles that attach at these impressions are three of the rotator cuff muscles:
The lateral aspect of the greater tubercle is covered by the deltoid muscle, which gives the shoulder its normal rounded shape. The lateral aspect of the greater tubercle is also marked by multiple vascular foramina.
The lesser tubercle is found anterior to the anatomical neck and has a smooth, palpable muscular impression. The lateral aspect of this tubercle forms the medial margin of the intertubercular sulcus. The lesser tubercle of the humerus gives attachment to the subscapularis muscle, and also to the transverse ligament of the shoulder.
Intertubercular sulcus - a deep groove separating the two tubercles. It is also known as the bicipital groove. The long tendon of the biceps brachii and a branch of the ascending circumflex humeral artery are situated within the intertubercular sulcus. The sulcus is formed by a lateral lip and a medial lip. The lateral lip is also known as the crest of the greater tubercle, it gives attachment to the tendon of the pectoralis major muscle. The medial lip serves as the attachment site for the teres major muscle. Also the tendon of the latissimus dorsi muscle attaches to its posterior aspect.
There is also a slight narrowing below the tubercles, which is known as the surgical neck of the humerus. It is a common fracture site. The axillary nerve and the posterior circumflex humeral artery (a branch of the axillary artery) lie close to the bone here. This is where the proximal extremity ends and continues as the shaft of the humerus.

Shaft of humerus

The shaft or body is the long middle part of the humerus and it gives attachment to several muscles. Cross section views reveal that the proximal half of the shaft is circular, while its distal half is triangular and flattened. The shaft of the humerus has three borders and three surfaces.

The borders of the shaft are as follows:

Anterior border - extends from the greater tubercle downward almost to the end of the humerus, its proximal end is continuous with the lateral lip of the intertubercular sulcus;
Lateral border - begins distal to the greater tubercle and it gets ticker distally to form the lateral supracondylar ridge, its middle portion is adjacent to the rough “V” shaped area called the deltoid tuberosity;
Medial border - forms the medial supracondylar ridge distally, it is interrupted in its medial third by the radial groove, where the radial nerve and the deep brachial artery (a branch of the axillary artery) are situated.

The surfaces of the shaft are the following:

Anterolateral surface - the area between the anterior and lateral borders of the shaft, its proximal surface is smooth and is mostly covered by the deltoid muscle; the deltoid inserts into the deltoid tuberosity, which is found around the middle of this surface; the lateral part of the brachialis originates from the distal part of the anterolateral surface, as well as from the proximal two-thirds of the lateral supracondylar ridge;
Anteromedial surface - the area between the anterior and medial borders; the coracobrachialis muscle attaches approximately in the mid-portion of this surface, while the distal half of the anteromedial surface is mostly covered by the medial portion of the brachialis muscle;
Posterior surface - located between the medial and lateral borders of the shaft and is covered mostly by the medial head of the triceps brachii, while a ridge on its proximal-third also gives attachment to the lateral head of the triceps brachii.

Distal end of humerus

The distal end (or distal extremity) of the humerus has both articular and non-articular parts.

The articular part is a modified condyle, which consists of a medial trochlea and a lateral capitulum, separated by a faint groove. This part articulates with both ulna and radius. The non-articular part of the distal end consists of the medial and lateral epicondyles, and several fossae - the olecranon fossa, coronoid fossa, and radial fossa.

The trochlea is a projection that is shaped like a pulley and located medially, extending onto the posterior aspect of the humerus. It articulates with the trochlear notch of the ulna.
Lateral to the trochlea, is the capitulum, which is a rounded and convex projection that articulates with the head of the radius.
The medial epicondyle is a blunt projection that forms the end of the medial border of the humerus. It is located superomedially to the medial condyle. The ulnar nerve passes in a groove on the posterior aspect of the medial epicondyle. The anterior surface of the medial epicondyle provides origin sites for the following superficial muscles of the anterior compartment of the forearm:
The lateral epicondyle is smaller than the medial one and it is found where the lateral border of the humerus ends. It is marked by an impression where the following seven superficial muscles of the posterior compartment of the forearm originate:
The olecranon fossa is a deep depression on the posterior surface of the distal end of the humerus, found superior to the trochlea. During extension of the elbow, the tip of the olecranon process of the ulna lodges into this fossa.
The coronoid fossa is a smaller depression that is also located superior to the trochlea, but on the anterior surface of the humerus. Upon elbow flexion, the coronoid process of the ulna lodges into this fossa.
Lateral to the coronoid fossa and superior to the capitulum of the humerus is another hallow area known as the radial fossa. The margin of the head of the radius lodges into this fossa during full elbow flexion.

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Humerus