The humerus is the only bone in the upper arm. Like many of the long bones in the body, the humerus has a proximal end, shaft, and distal end. Proximal refers to the area closest to the center of the body -- the shoulder. Distal is the end of the bone closest to the elbow.
Long bones like the humerus support a significant load while the arm carries out various functions, and has a specific internal structure. The outside of the bone has a layer of connective tissue -- the periosteum. The outer layer of the bone itself is hard and compact, surrounding an internal layer of spongy cancellous bone. The innermost medullary cavity contains the bone marrow.
The proximal end of the humerus begins with the head, which expands in all directions from the slimmer anatomical neck. On one side of the bone is the greater tubercle, a bony projection for muscle attachment. The lesser tubercle is smaller and sits closer to the center of the humerus. Between the two tubercles is the deep groove of the intertubercular sulcus. The surgical neck of the humerus lies just underneath the tubercles.
Between the proximal and distal ends of the humerus is the shaft. This section of the bone has many muscle attachment sites. A cross-section of the bone reveals the shaft is circular on the proximal end and flatter towards the distal end. Near the middle of the shaft is a rough surface -- the deltoid tuberosity -- that acts as an attachment point for the deltoid muscle. A shallow indentation, the radial groove, stretches along the back of the humerus.
Like the proximal end, the distal portion of the humerus has many unique bony landmarks. The borders or edges of the humerus make up the lateral supracondylar ridges, which have rough surfaces for muscle attachment. Just below the ridges are bony projections, the lateral and medial epicondyles. The medial epicondyle is the larger of the two and extends further. Toward the center of the humerus are the trochlea and the capitulum. The distal end is also home to three depressions: the olecranon, radial, and coronoid fossae.
The humerus forms two joints. At the distal end, it articulates or moves in concert with the radius and the ulna to form the elbow joint. Specifically, the head of the radius articulates with the capitulum of the humerus. The trochlea also articulates with the trochlear notch of the ulna. The elbow is a hinge joint, meaning that it is only capable of extending or flexing the forearm.
On the proximal end, the humerus helps form the shoulder joint, anatomically known as the glenohumeral joint, in reference to the bone sections that articulate to form it: the head of the humerus and the glenoid fossa of the scapula. Unlike the elbow, the shoulder is a ball and socket joint. This means it is capable of a much larger range of motion with much more variation. It is one of the most mobile joints in the human body, though the mobility comes at the cost of reduced stability.
Many important nerves travel along the humerus to the rest of the arm and hand. On the proximal end, the surgical neck is home to the axillary nerve, which wraps around the bone. At the shaft, the radial groove hosts the radial nerve. The distal end has a groove along the back of the medial epicondyle, where the ulnar nerve passes. Because the ulnar nerve has no bone or muscle protecting it, it is susceptible to damage. This is the cause of the electric sensation that many people refer to as striking the “funny bone.”
Many sites on the humerus act as muscle attachment sites.
During embryo development, the humerus is one of the first structures to begin ossification -- the process by which bones harden. At birth, only the shaft has undergone ossification. The head begins to ossify around four months, and the greater tuberosity begins the process at ten months. Generally, the process finishes by the time a child reaches 13 years.
Many factors lead to the humerus being one of the most-fractured bones. It is responsible for supporting all lifting activities, it lacks a supporting bone, and it is in a location prone to trauma. Fractures can occur almost anywhere on the bone, and falls are responsible for most humerus breaks. In some cases, fractures can damage or inhibit the nerves that run along the humerus. Some fractures can cause arm deformities that require surgery.
This site offers information designed for educational purposes only. You should not rely on any information on this site as a substitute for professional medical advice, diagnosis, treatment, or as a substitute for, professional counseling care, advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a physician or other healthcare professional.