External intercostal muscles

The external intercostal muscles are the most superficial layer of the intercostal muscle group, lying between the ribs in each intercostal space. These muscles are primarily responsible for elevating the ribs during inspiration, contributing to the expansion of the thoracic cavity and facilitating airflow into the lungs. They are essential for maintaining the structural integrity of the thoracic wall and play an important role in the mechanics of breathing.

Location

External intercostal muscles occupy the eleven intercostal spaces on each side of the thoracic cage. They extend from the rib tubercles posteriorly to the costochondral junctions anteriorly, where they are replaced by a thin fibrous sheet known as the external intercostal membrane. The muscles lie deep to the serratus anterior and superficial to the internal intercostal muscles.

Origin and Insertion

• Origin: Inferior border of the rib above
• Insertion: Superior border of the rib below

Each muscle runs obliquely downward and forward, forming a slanted, diagonal pattern that resembles the direction of placing hands into one's pockets. This fiber orientation contributes to rib elevation during contraction.

Fiber Direction

The muscle fibers run in an inferomedial direction—downward and toward the midline—at approximately a 45-degree angle. This direction is opposite to that of the internal intercostal muscles, which run inferolaterally.

Extent and Boundaries

• Posterior boundary: Rib tubercles
• Anterior boundary: Costochondral junctions (beyond this point, replaced by the external intercostal membrane)

The external intercostal membrane continues anteriorly from the muscle fibers toward the sternum and contributes to the overall elasticity of the thoracic wall.

Layer Relationships

The external intercostal muscles are the outermost layer in the intercostal space. From superficial to deep, the arrangement is:

1. External intercostal muscle
2. Internal intercostal muscle
3. Innermost intercostal muscle

The intercostal neurovascular bundle (vein, artery, nerve) runs between the internal and innermost layers within the costal groove.

Function

The primary function of the external intercostal muscles is to elevate the ribs during inspiration. This action increases the anteroposterior and transverse dimensions of the thoracic cavity, reducing intrathoracic pressure and allowing air to flow into the lungs.

• Participate in quiet and forced inspiration
• Stabilize intercostal spaces and rib margins during movement and breathing
• Assist in maintaining thoracic wall integrity

During breathing, the coordinated contraction of the external intercostals with the diaphragm ensures smooth expansion of the thoracic volume, optimizing ventilatory efficiency.

Role in Respiratory Mechanics

External intercostals act synergistically with other respiratory muscles:

• They work alongside the diaphragm during quiet inspiration
• In forced inspiration, they assist accessory muscles such as sternocleidomastoid and scalene muscles

The upward and outward movement of the ribs they produce contributes to the "bucket-handle" and "pump-handle" mechanisms of rib motion.

Blood Supply

• Arterial: Supplied by anterior intercostal arteries (branches of the internal thoracic and musculophrenic arteries) and posterior intercostal arteries (branches of the thoracic aorta)
• Venous: Drained by anterior and posterior intercostal veins into the internal thoracic and azygos systems

Innervation

• Innervated by the intercostal nerves (anterior rami of thoracic spinal nerves T1–T11)

Each intercostal nerve provides motor supply to the corresponding intercostal muscles and sensory fibers to the overlying skin and parietal pleura.

Embryological Origin

External intercostal muscles develop from the myotomes of the paraxial mesoderm, specifically from thoracic somites. Their segmental innervation reflects this origin.

Comparative Anatomy with Other Intercostals

• Internal intercostals: Lie deep to the external intercostals; depress ribs during forced expiration; fibers run at right angles
• Innermost intercostals: Deepest layer; reinforce rib cage structure and assist in expiration

Together, all three layers help regulate thoracic pressure changes during respiration and support thoracic wall tension.

Clinical Relevance

• Muscle strain: External intercostals may be strained during excessive twisting or deep breathing; presents as localized pain
• Chest tube placement: Knowledge of intercostal muscle layers is vital; tubes are inserted above the superior border of a rib to avoid damaging the neurovascular bundle