The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a rounded head, a narrow neck, and two short processes (tubercles, sometimes called tuberosities). The shaft is cylindrical in its upper portion, and more prismatic below. The lower extremity consists of 2 epicondyles, 2 processes (trochlea and capitulum), and 3 fossae (radial fossa, coronoid fossa, and olecranon fossa). As well as its true anatomical neck, the constriction below the greater and lesser tubercles of the humerus is referred to as its surgical neck due to its tendency to fracture, thus often becoming the focus of surgeons.

Etymology

The word "humerus" is derived from Late Latin , from Latin , meaning upper arm, shoulder, and is linguistically related to Gothic (shoulder) and Greek .

Structure

Upper extremity

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====<!-- Head of the humerus redirects here -->

right|thumb|Head of the humerus

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). The circumference of its articular surface is slightly constricted and is termed the anatomical neck, in contradistinction to a constriction below the tubercles called the surgical neck which is frequently the seat of fracture. Fracture of the anatomical neck rarely occurs.

The diameter of the humeral head is generally larger in men than in women.

Anatomical neck

The anatomical neck (collum anatomicum) is obliquely directed, forming an obtuse angle with the body. It is most prominent in the lower half of its circumference, while in the upper half, it is represented by a narrow groove separating the head from the tubercles. The line separating the head from the rest of the upper end is called the anatomical neck. It affords attachment to the articular capsule of the shoulder-joint, and is perforated by numerous vascular foramens. Fracture of the anatomical neck rarely occurs.

Borders

The medial portion of the articular surface is named the trochlea, and presents a deep depression between two well-marked borders; it is convex from before backward, concave from side to side, and occupies the anterior, lower, and posterior parts of the extremity.

  • The lateral border separates it from the groove which articulates with the margin of the head of the radius.
  • The medial border is thicker, of greater length, and consequently more prominent, than the lateral.

The grooved portion of the articular surface fits accurately within the semilunar notch of the ulna; it is broader and deeper on the posterior than on the anterior aspect of the bone, and is inclined obliquely downward and forward toward the medial side.

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. Dislocation of the humerus's glenohumeral joint has the potential to injure the axillary nerve or the axillary artery. Signs and symptoms of this dislocation include a loss of the normal shoulder contour and a palpable depression under the acromion.

The radial nerve follows the humerus closely. At the midshaft of the humerus, the radial nerve travels from the posterior to the anterior aspect of the bone in the spiral groove. A fracture of the humerus in this region can result in radial nerve injury.

The ulnar nerve lies at the distal end of the humerus near the elbow. When struck, it can cause a distinct tingling sensation, and sometimes a significant amount of pain. It is sometimes popularly referred to as 'the funny bone', possibly due to this sensation (a "funny" feeling), as well as the fact that the bone's name is a homophone of 'humorous'. It lies posterior to the medial epicondyle, and is easily damaged in elbow injuries.

Function

Muscular attachment

The deltoid originates on the lateral third of the clavicle, acromion and the crest of the spine of the scapula. It is inserted on the deltoid tuberosity of the humerus and has several actions including abduction, extension, and circumduction of the shoulder. The supraspinatus also originates on the spine of the scapula. It inserts on the greater tubercle of the humerus, and assists in abduction of the shoulder.

The pectoralis major, teres major, and latissimus dorsi insert at the intertubercular groove of the humerus. They work to adduct and medially, or internally, rotate the humerus.

The infraspinatus and teres minor insert on the greater tubercle, and work to laterally, or externally, rotate the humerus. In contrast, the subscapularis muscle inserts onto the lesser tubercle and works to medially, or internally, rotate the humerus.

The biceps brachii, brachialis, and brachioradialis (which attaches distally) act to flex the elbow. (The biceps do not attach to the humerus.) The triceps brachii and anconeus extend the elbow, and attach to the posterior side of the humerus.

The four muscles of supraspinatus, infraspinatus, teres minor and subscapularis form a musculo-ligamentous girdle called the rotator cuff. This cuff stabilizes the very mobile but inherently unstable glenohumeral joint. The other muscles are used as counterbalances for the actions of lifting/pulling and pressing/pushing.

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|+ Left humerus. Posterior view.

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Other animals

Primitive fossils of amphibians had little, if any, shaft connecting the upper and lower extremities, making their limbs very short. In most living tetrapods, however, the humerus has a similar form to that of humans. In many reptiles and some mammals (where it is the primitive state), the lower extremity includes a large opening called the entepicondylar foramen to allow the passage of nerves and blood vessels.

Additional images

<gallery>

File:Humerus animation.gif|Position of humerus (shown in red). Animation.

File:Left humerus - close-up - animation.gif|Left humerus. Animation.

File:Human humerus 2.stl|3D image

File:Human arm bones diagram.svg|Human arm bones diagram.

File:Inferior epiphysis of humerus.jpg|Humerus - inferior epiphysis. Anterior view.

File:Slide10DEN.JPG|Trochlea. Posterior view.

File:LeftHumanPosteriorDistalHumerusFlexed.jpg|Humerus - inferior epiphysis. Posterior view.

File:AnteroMedialHeadofLeftHumerus.jpg|Humerus - superior epiphysis. Anterior view.

File:Superior epiphysis.jpg|Humerus - superior epiphysis. Posterior view.

File:Elbow joint - deep dissection (anterior view, human cadaver).jpg|Elbow joint. Deep dissection. Anterior view.

File:Slide1bgbg.JPG|Elbow joint. Deep dissection. Posterior view.

File:Slide3bgbg.JPG|Elbow joint. Deep dissection. Posterior view.

File:Gray326.png|The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula

File:Gray207.png|Left humerus. Anterior view.

File:Gray208.png|Left humerus. Posterior view.

File:AnteriorHeadOfLeftHumerus.jpg|Left humerus. Anteriolateral view.

File:MedialHeadOfLeftHumerus.jpg|Left humerus. Medial view.

File:ProxHumeralFracture.png|Fracture of the proximal humerus

File:Gray329.png|Left elbow-joint, showing anterior and ulnar collateral ligaments.

File:Gray332.png|Capsule of elbow-joint (distended). Posterior aspect.

File:Humerus Anatomy by Jason Christian.webm|Humerus anatomy

</gallery>

Ossification

During embryonic development, the humerus is one of the first structures to ossify, beginning with the first ossification center in the shaft of the bone. Ossification of the humerus occurs predictably in the embryo and fetus, and is therefore used as a fetal biometric measurement when determining gestational age of a fetus. At birth, the neonatal humerus is only ossified in the shaft. The epiphyses are cartilaginous at birth. The medial humeral head develops an ossification center around 4 months of age and the greater tuberosity around 10 months of age. These ossification centers begin to fuse at 3 years of age. The process of ossification is complete by 13 years of age, though the epiphyseal plate (growth plate) persists until skeletal maturity, usually around 17 years of age.

See also

  • Humerus fracture

References

  • Humerus - BlueLink Anatomy, University of Michigan Medical School