arm - Anatomy of the human arm

A term commonly used to denote the whole of the upper limb; more precisely, in anatomy, the region between the shoulder and elbow joints, distinguished from the forearm (between the elbow and wrist joints), and the hand (beyond the wrist joint). The arm articulates with the trunk via the pectoral girdle (the scapula, or shoulder blade, and clavicle). The bones are the humerus in the arm, the radius (on the outside) and ulna (on the inside) in the forearm, the carpals in the wrist region, and the metacarpals and phalanges in the hand. The muscles on the front of the arm and forearm cause flexion of the shoulder, elbow, wrist, and digits (fingers and thumb). The muscles on the back of the arm and forearm are extensors, causing opposite movements at these places.

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Anatomy of the human arm

The human arm contains bones, joints, muscles, nerves, and blood vessels.

Bony structure and joints

The humerus is the (upper) arm bone. fdfThe elbow joint fbfdifdb

Osteofascial compartments

The arm is divided by a fascial layer (known as lateral and medial intermuscular septa) separating the muscles into an anterior and posterior osteofascial compartments. The compartments contain muscles which are innervated by the same nerve and perform the same action. They are all supplied by the musculocutaneous nerve, which arises from the fifth, sixth, and seventh cervical spinal nerves (see Brachial plexus).

The large deltoid muscle is considered to have part of its body in the anterior compartment. This muscle is the main adductor muscle of the upper limb and extends over the shoulder.

The brachioradialis muscle originates in the arm but inserts into the forearm.

The posterior compartment contains muscles which are all supplied by the radial nerve. The muscles of this compartment are the triceps brachii and anconeus muscle. The triceps brachii is a large muscle containing three heads (lateral, medial, and middle). As the upper and lower limbs have similar embryological origins and the lower limb contains the quadriceps femoris muscle (the lower limb equivalent of the triceps), which has four heads, this would seem to make sense. It is an imaginary triangle with borders being:

Laterally, the medial border of brachioradialis muscle Medially, the lateral border of pronator teres muscle Superiorly, the intercondylar line, an imaginary line between the two condyles of the humerus The floor is the brachialis muscle The roof is the skin and fascia of the arm and forearm

The structures which pass through the cubital fossa are vital. The order from which they pass into the forearm are as follows, from medial to lateral:

Median nerve, which starts to branch Brachial artery Tendon of the biceps brachii muscle Radial nerve Median cubital vein - this important vein is where venepuncture occurs. lymph nodes

Nervous supply

The musculocutaneous nerve, from C5, C6, C7, is the main supplier of muscles of the anterior compartment. It originates from the lateral cord of the brachial plexus of nerves. It pierces the coracobrachialis muscle and gives off branches to the muscle, as well as to brachialis and biceps brachii. It terminates as the anterior cutaneous nerve of the forearm.

The radial nerve, which is from the fifth cervical spinal nerve to the first thoracic spinal nerve, originates as the continuation of the posterior cord of the brachial plexus. This nerve enters the lower triangular space (an imaginary space bounded by, amongst others, the shaft of the humerus and the triceps brachii) of the arm and lies deep to the triceps brachii. This fact is very important clinically as a fracture of the bone at the shaft of the bone here can cause lesions or even transections in the nerve.

Other nerves passing through give no supply to the arm. These include:

The median nerve, nerve origin C5-T1, which is a branch of the lateral and medial cords of the brachial plexus. This nerve continues in the arm, travelling in a plane between the biceps and triceps muscles. At the cubital fossa, this nerve is deep to the pronator teres muscle and is the most medial structure in the fossa. The ulnar nerve, origin C7-T1, is a continuation of the medial cord of the brachial plexus. This nerve passes in the same plane as the median nerve, between the biceps and triceps muscles. At the elbow, this nerve travels posterior to the medial epicondyle of the humerus.

Blood supply and venous drainage

The main artery in the arm is the brachial artery. The brachial artery gives off an important brach, the profunda brachii (deep artery of the arm).

The brachial artery continues to the cubital fossa in the anterior compartment of the arm. It travels in a plane between the biceps and triceps muscles, the same as the median nerve and basilic vein. The artery is in between the median nerve and the tendon of the biceps muscle in the cubital fossa.

The profunda brachii travels through the lower triangular space with the radial nerve. Therefore fracture of the bone may not only lead to lesion of the radial nerve, but also haematoma of the internal structures of the arm.

The veins of the arm carry blood from the extremities of the limb, as well as drain the arm itself.

The basilic vein travels on the medial side of the arm and terminates at the level of the seventh rib.

The cephalic vein travels on the lateral side of the arm and terminates as the axillary vein.