The lateral thorax, often called the side of the chest, extends from the armpit down to the lower margins of the rib cage. This complex wall of muscle, bone, and connective tissue provides protection for internal organs and acts as a functional bridge between the trunk and the mobile upper limb.
Key Anatomical Structures of the Lateral Thorax
The bony framework is defined by the curvature of the ribs, which arch forward from the spine. Between the ribs lie the external and internal intercostal muscles, forming a continuous muscular layer. These muscles contain the neurovascular bundle (intercostal nerve, artery, and vein), which runs along the groove on the underside of each rib.
The Serratus Anterior is a large, flat muscle lying directly against the ribs. It originates from the first eight or nine ribs and attaches to the inner surface of the scapula (shoulder blade). Due to its role in pushing and punching motions, it is often nicknamed the “boxer’s muscle.”
The Long Thoracic Nerve originates from the spinal nerves of the neck (C5, C6, and C7). It travels a long, superficial course down the side of the chest wall to exclusively supply the Serratus Anterior muscle. This exposed path makes the nerve highly susceptible to injury.
The blood supply to the area is primarily managed by the Lateral Thoracic Artery and its accompanying vein. The artery branches off the axillary artery in the armpit and descends along the side of the chest. It supplies oxygenated blood to the Serratus Anterior and the pectoralis muscles, and also gives off lateral mammary branches in females.
Sensory input from the skin covering the lateral thorax is handled by the lateral cutaneous branches of the intercostal nerves. These nerves pierce through the intercostal muscles and the Serratus Anterior muscle near the mid-axillary line. Once through the muscle layers, they divide into anterior and posterior branches to provide feeling to the skin of the side and front of the chest.
Essential Roles in Movement and Respiration
The lateral thoracic muscles enable both arm movement and fundamental breathing mechanics. The Serratus Anterior acts as a powerful stabilizer and mover of the scapula. This muscle holds the medial border of the scapula tightly against the rib cage, preventing it from lifting away during arm movement.
When the arm is raised overhead, the Serratus Anterior works in coordination with the trapezius muscle to upwardly rotate the scapula. This synchronized movement is known as the scapulohumeral rhythm. This rhythm is necessary to achieve the full range of motion for reaching above the head. The action of pulling the scapula forward around the chest wall is called protraction, which is essential for pushing or throwing motions.
The intercostal muscles between the ribs are dedicated to the process of breathing. The external intercostals are responsible for elevating the ribs during inhalation, which expands the chest cavity volume. Conversely, the internal intercostals contribute to depressing the ribs during forced or heavy exhalation.
The Serratus Anterior also assists in breathing by acting as an accessory muscle of respiration. When the shoulder girdle is fixed, its contraction helps elevate the ribs during deep or labored breathing. The intercostal muscles primarily expand and contract the rib cage, but they also provide rigidity to the thoracic wall. This stiffness prevents the rib cage from collapsing inward due to pressure changes during respiration.
The intercostals also assist in general trunk motion, working in conjunction with the larger abdominal muscles. While the external and internal oblique muscles are the primary movers for trunk rotation and side bending, the intercostals assist in stabilizing the rib cage during these powerful movements. Their contribution ensures that the bony structure of the chest provides a stable base for movement.
Common Injuries and Pathologies
Damage to the Long Thoracic Nerve leads to Long Thoracic Nerve Palsy. Injury to this nerve results in paralysis or weakness of the Serratus Anterior muscle, which is the sole muscle it supplies. The resulting functional deficit is known as “winged scapula.”
Scapular winging occurs because the Serratus Anterior can no longer anchor the scapula against the chest wall, causing the bone to protrude noticeably, especially when the person pushes against a wall. This loss of stability severely limits the ability to lift the arm above shoulder height. Causes of Long Thoracic Nerve Palsy include blunt trauma, repetitive stretching during athletic activity, or complications from surgical procedures like mastectomy.
Another common source of discomfort in this region is Intercostal Neuralgia, which is pain that follows the path of one or more intercostal nerves. This neuropathic pain is often described as a sharp, stabbing, or burning sensation that wraps around the chest or rib cage. The pain is frequently aggravated by movements that stretch the nerves, such as deep breathing, coughing, or laughing.
Intercostal Neuralgia can arise from several causes, including physical trauma (like a rib fracture), nerve compression, or a viral infection. A notable cause is the reactivation of the varicella-zoster virus (shingles), where nerve inflammation causes pain that can persist after the rash resolves.
The muscles of the lateral thorax are also prone to strains and contusions, particularly the Serratus Anterior and the intercostals themselves. Serratus Anterior strains are often seen in athletes involved in repetitive overhead or throwing motions, such as baseball pitchers or swimmers. These injuries manifest as tenderness and pain along the side of the rib cage, often making deep inspiration or forceful arm movements painful.
Trauma to the lateral chest wall, such as from a car accident or a fall, commonly results in rib fractures, which directly affect the underlying structures. A fractured rib can cause acute pain and significantly impair respiration by limiting the ability of the intercostal muscles to move the rib cage. The sharp edges of a broken rib can also directly irritate or lacerate the intercostal nerve and vessels, further contributing to pain and potential nerve damage.