Arm

The arm pertains to the part between the shoulder and the elbow.

Bony structure:

• The humerus is the main bone of the arm.

• Proximally, its head connects with the glenoid cavity of the scapula, forming the shoulder joint (a.k.a. the glenohumeral joint). It is a ball and socket variety of synovial joint that permits a wide range of movements including flexion, extension, adduction, abduction, and circumduction.

• Distally, the rounded capitulum of the humerus articulates with the head of the radius, while the trochlea of the humerus, shaped like a pulley, fits into the trochlear notch of ulna, creating the elbow joint. It is a hinge joint that facilitates flexion and extension movements.

Muscles:

The muscles covering the bony structure of the arm can be divided into anterior (or flexor) and posterior (or extensor) compartments. The anterior compartment includes the coracobrachialis, biceps brachii, and brachialis muscles, while the posterior compartment consists of the triceps brachii muscle.

• Coracobrachialis originates from the coracoid process of the scapula, crosses the shoulder joint, and inserts into the mid-shaft of the humerus, flexing the arm at the shoulder joint.

• Biceps brachii has short and long heads. The short head originates from the coracoid process of the scapula, while the long head originates from the supraglenoid tubercle located just above the glenoid cavity. Both heads merge to form a single muscle belly, cross the elbow joint, and insert into the radial tuberosity (the upper end of the radius), allowing for forearm flexion at the elbow joint. It also supinates the forearm at the superior radioulnar joint.

• Brachialis originates from the ventral surface of the humerus, crosses the elbow joint, and inserts into the ulnar tuberosity, enabling forearm flexion at the elbow joint.

• Triceps brachii has three heads: long, medial, and lateral. The long head originates from the infraglenoid tubercle of the scapula just below the glenoid cavity, while the medial and lateral heads arise from the posterior surface of the humerus. All heads converge to form a single muscle belly, cross the dorsal aspect of the elbow joint, and insert into the olecranon (at the proximal end of the ulna), extending the forearm at the elbow joint.

Nerves:

The arm’s nerves consist of distal branches stemming from the brachial plexus, which include the median, ulnar, musculocutaneous, and radial nerves.

• The musculocutaneous nerve emanates from the lateral cord of the brachial plexus and innervates all flexor muscles of the arm.

• The radial nerve arises from the posterior cord of the brachial plexus. It innervates the triceps brachii and the extensor muscles of the forearm.

• The median nerve (from the lateral and medial cords) and the ulnar nerve (from the medial cord), course through the anterior arm. However, they don’t innervate any flexor muscles of the arm.

• The median nerve primarily innervates the muscles of the anterior forearm, while the ulnar nerve predominantly innervates the muscles of the palm.

Vasculature:

• The vasculature of the arm consists of the brachial artery, which continues distally from the axillary artery and bifurcates into the radial and ulnar arteries in the cubital fossa (located in front of the elbow joint).

• The deep veins accompany the brachial artery, while the superficial veins, originating from the venous arch on the dorsum (back) of the hand, comprise the cephalic vein (on the radial side) and the basilic vein (on the ulnar side). These veins traverse the arm and eventually drain into the deep brachial veins.

Fascia and skin:

• Comparable to other body regions, the arm and upper limb are enveloped by deep and superficial facia, along with the skin.