External Intercostals

The external intercostal muscles are the most superficial of the three principal muscular layers occupying the intercostal spaces. They extend between adjacent ribs and play an important role in maintaining the structural integrity of the thoracic wall and assisting inspiration.

During inspiration, contraction of the external intercostals helps elevate the ribs and stabilize the intercostal spaces. Rib elevation increases the anteroposterior and transverse dimensions of the thoracic cavity, complementing the vertical expansion produced by contraction of the diaphragm.

Location

The external intercostal muscles occupy the intercostal spaces between adjacent ribs.

They form the superficial muscular layer of each intercostal space and lie superficial to the internal intercostal muscles.

Extent

External intercostal muscle fibers extend from the region of the rib tubercles posteriorly toward the costochondral junctions anteriorly.

Anterior to the costochondral junctions, the muscular layer is replaced by the external intercostal membrane, which continues toward the sternum.

Attachments

Each external intercostal muscle extends from the inferior border of the rib above to the superior border of the rib below.

This arrangement links adjacent ribs throughout much of the posterolateral thoracic wall.

Fiber Direction

The fibers of the external intercostals pass generally inferoanteriorly from the rib above to the rib below.

This orientation is often described as resembling the direction of the fingers when the hands are placed in the pockets of a jacket.

Comparison with Internal Intercostals

Feature	External Intercostals	Internal Intercostals
Layer	Superficial	Middle
Fiber direction	Inferoanterior	Generally inferoposterior
Anterior extent	To costochondral junction, then membrane	Extends anteriorly between costal cartilages
Major respiratory role	Assists inspiration	Interosseous portion assists forced expiration; interchondral portion can assist inspiration

Role in Inspiration

The external intercostals are primarily inspiratory muscles. Their coordinated contraction helps elevate adjacent ribs and maintain the spacing and rigidity of the thoracic wall during inspiration.

This rib movement contributes to expansion of the thoracic cavity and the pressure changes that draw air into the lungs.

Rib Elevation

When the superior ribs are stabilized, contraction of the external intercostals can exert an upward force on the ribs below.

Coordinated activity across multiple intercostal spaces contributes to elevation of the rib cage rather than isolated movement of individual ribs.

Thoracic Expansion

Elevation of the ribs increases the dimensions of the thoracic cavity.

The resulting expansion contributes to a decrease in intrapleural pressure and expansion of the lungs during inspiration.

Pump-Handle Movement

The upper ribs and sternum move in a manner commonly described as a pump-handle movement.

Elevation of these structures moves the sternum anteriorly and superiorly, increasing the anteroposterior diameter of the thoracic cavity.

Bucket-Handle Movement

The middle and lower ribs also move laterally during elevation.

This bucket-handle movement increases the transverse diameter of the thoracic cavity and contributes to inspiratory expansion.

Relationship to the Diaphragm

The diaphragm is the principal muscle of inspiration and increases the vertical dimension of the thoracic cavity as it contracts and descends.

External intercostal contraction complements diaphragmatic action by expanding and stabilizing the rib cage.

Quiet Inspiration

During quiet breathing, diaphragmatic contraction provides most of the inspiratory force.

The external intercostals contribute to rib movement and thoracic wall stabilization, with their degree of recruitment varying according to posture, respiratory depth, and mechanical demand.

Deep Inspiration

During deeper inspiration, greater rib elevation is required to expand the thoracic cavity.

External intercostal activity can increase together with recruitment of accessory inspiratory muscles.

Forced Inspiration

During forceful inspiration, the external intercostals work with the diaphragm and accessory inspiratory muscles to produce greater thoracic expansion.

Accessory muscles may elevate the sternum and upper ribs while the external intercostals coordinate movement and stabilization across the intercostal spaces.

Thoracic Wall Stabilization

An important function of the external intercostals is stabilization of the intercostal spaces during respiratory pressure changes.

Without muscular support, the soft tissues between adjacent ribs would be more susceptible to deformation as pressure within the thorax changes.

Maintenance of Intercostal Space Integrity

Coordinated intercostal muscle contraction stiffens the thoracic wall while still permitting rib movement.

This allows respiratory forces to be transmitted efficiently through the rib cage during ventilation.

External Intercostal Membrane

Anteriorly, external intercostal muscle fibers terminate near the costochondral junctions.

The layer continues toward the sternum as the external intercostal membrane, a thin fibrous sheet occupying the anterior part of the intercostal space.

Posterior Extent

The external intercostals extend farther posteriorly than the internal intercostals.

The muscular fibers reach approximately to the rib tubercles near the vertebral column.

Intercostal Muscle Layers

The thoracic wall contains three principal intercostal muscular layers:

1. External intercostals
2. Internal intercostals
3. Innermost intercostals

These layers differ in fiber direction, extent, anatomical relationships, and respiratory function.

Relationship to Internal Intercostals

The internal intercostal muscles lie immediately deep to the external intercostals.

The two layers have approximately perpendicular fiber orientations and contribute differently to thoracic mechanics.

Relationship to Innermost Intercostals

The innermost intercostal muscles form the deepest discontinuous muscular layer of the intercostal spaces.

The major intercostal neurovascular bundle lies in the plane between the internal and innermost intercostal layers, deeper than the external intercostals.

Intercostal Neurovascular Bundle

The principal intercostal vein, artery, and nerve travel along the inferior margin of the rib above, primarily within the costal groove.

They lie in the neurovascular plane between the internal and innermost intercostal muscle layers.

Neurovascular Arrangement

The principal structures in the costal groove are generally arranged from superior to inferior as:

1. Intercostal vein
2. Intercostal artery
3. Intercostal nerve

This arrangement is commonly abbreviated as VAN.

Innervation

The external intercostal muscles are innervated by the intercostal nerves, which are derived from the anterior rami of thoracic spinal nerves.

Innervation is segmental, with each intercostal nerve supplying muscles within its corresponding intercostal space.

Intercostal Nerves

Intercostal nerves travel around the thoracic wall within their respective intercostal spaces.

They provide motor fibers to intercostal muscles and sensory fibers to the thoracic wall and associated structures.

Blood Supply

The external intercostal muscles receive blood from the anterior and posterior intercostal arteries.

These vessels form extensive anastomoses within the intercostal spaces.

Posterior Intercostal Arteries

Most posterior intercostal arteries arise directly from the thoracic aorta.

The uppermost spaces receive arterial supply through branches associated with the supreme intercostal artery.

Anterior Intercostal Arteries

Anterior intercostal arteries arise from the internal thoracic artery and its musculophrenic branch.

They pass laterally and anastomose with posterior intercostal arteries within the thoracic wall.

Venous Drainage

Intercostal veins accompany the corresponding arteries and nerves.

Posterior intercostal veins drain predominantly toward the azygos and hemiazygos venous systems, while anterior intercostal veins communicate with the internal thoracic venous system.

Mechanical Principle of Rib Movement

Because external intercostal fibers connect adjacent ribs obliquely, their contraction generates forces between neighboring ribs.

When the upper thoracic framework is stabilized, the net coordinated action favors elevation of the rib cage during inspiration.

Upper Rib Stabilization

The first rib can be stabilized by muscles such as the scalene muscles during increased inspiratory effort.

This provides a relatively fixed superior point from which coordinated intercostal contraction can contribute to elevation of more inferior ribs.

Relationship to Accessory Inspiratory Muscles

During increased respiratory demand, external intercostals work with accessory inspiratory muscles.

The sternocleidomastoid, scalenes, pectoral muscles, and other muscles can assist thoracic expansion when their appropriate attachments are stabilized.

Inspiration and Pressure Changes

Expansion of the thoracic cavity causes intrapleural pressure to become more negative.

The lungs expand with the thoracic wall through pleural coupling, lowering alveolar pressure relative to atmospheric pressure and allowing air to enter the respiratory tract.

Sequence During Inspiration

1. The diaphragm contracts and descends.
2. External intercostals contribute to rib elevation and stabilization.
3. The sternum and ribs move superiorly and outward.
4. The vertical, anteroposterior, and transverse dimensions of the thorax increase.
5. Intrapleural pressure becomes more negative.
6. The lungs expand.
7. Alveolar pressure falls below atmospheric pressure.
8. Air flows into the lungs.

Expiration

During quiet expiration, inspiratory muscle activity decreases and the respiratory system returns toward its resting configuration largely through elastic recoil.

The external intercostals therefore primarily participate in inspiration rather than active expiration.

Relationship to Forced Expiration

Forced expiration recruits muscles that actively decrease thoracic volume.

These include the interosseous portions of the internal intercostals and the abdominal muscles rather than the external intercostals.

Role During Exercise

Exercise increases the volume and rate of ventilation.

Greater inspiratory demand can increase recruitment of the external intercostals together with the diaphragm and accessory inspiratory muscles.

Role During Deep Breathing

During deep voluntary inspiration, the rib cage expands more extensively than during ordinary tidal breathing.

External intercostal contraction contributes to this increased rib elevation and stabilization.

Role in Respiratory Distress

When the work of breathing increases, inspiratory muscles may be recruited more strongly.

External intercostal activity works alongside the diaphragm and accessory inspiratory muscles to support thoracic expansion.

Intercostal Muscle Weakness

Weakness of the intercostal muscles can reduce the ability to stabilize and move the thoracic cage effectively.

The degree of respiratory impairment depends on the severity of weakness and the functional status of the diaphragm and other respiratory muscles.

Spinal Cord Injury

Thoracic spinal cord injury can impair neural pathways controlling intercostal musculature.

Loss of intercostal muscle activity can reduce chest wall stability and alter respiratory mechanics even when diaphragmatic function remains intact.

Intercostal Nerve Injury

Damage to an intercostal nerve may impair muscle function within the corresponding thoracic segment.

Possible causes include trauma, surgery, compression, and other lesions affecting the thoracic wall or spinal nerve pathways.

Chest Wall Trauma

Injuries involving the ribs and intercostal muscles can interfere with normal chest wall mechanics.

Pain can also limit deep inspiration, reducing normal rib movement even when the muscles and nerves remain structurally intact.

Thoracic Procedures

Knowledge of intercostal muscle layers is important during procedures that traverse the thoracic wall.

The relationship between the ribs, intercostal muscles, pleura, and neurovascular bundles helps guide approaches designed to minimize injury to important structures.

Intercostal Spaces

Each intercostal space contains muscular, vascular, and neural structures arranged between adjacent ribs.

The external intercostal muscles form the superficial muscular component of this layered thoracic wall organization.

Comparison of Intercostal Layers

Layer	Fiber Direction	Extent	General Respiratory Role
External intercostals	Inferoanterior	Rib tubercles to costochondral junctions	Assist inspiration and stabilize ribs
Internal intercostals	Inferoposterior	Sternum to rib angles	Interosseous portion assists forced expiration; interchondral portion can assist inspiration
Innermost intercostals	Similar to internal layer	Discontinuous	Assist internal intercostal function and thoracic wall stabilization

Key Anatomical Relationships

• External intercostals are the superficial intercostal muscle layer.
• They occupy the spaces between adjacent ribs.
• Their fibers generally pass inferoanteriorly.
• They extend from approximately the rib tubercles posteriorly to the costochondral junctions anteriorly.
• Anteriorly, the layer continues as the external intercostal membrane.
• They lie superficial to the internal intercostal muscles.
• Their principal respiratory role is assisting inspiration.
• They elevate and stabilize the ribs during inspiratory activity.
• Rib elevation increases the anteroposterior and transverse dimensions of the thorax.
• They work with the diaphragm during inspiration.
• They are innervated segmentally by intercostal nerves.
• They receive blood from anterior and posterior intercostal arteries.
• The main intercostal neurovascular bundle lies deeper, between the internal and innermost intercostal layers.
• Greater recruitment can occur during deep or forceful inspiration.

Clinical Importance

The external intercostal muscles are important components of the thoracic wall and contribute to both respiratory movement and mechanical stabilization. Their oblique fibers link adjacent ribs and help coordinate elevation of the rib cage during inspiration.

Their action complements the diaphragm. While diaphragmatic contraction primarily increases the vertical dimension of the thorax, external intercostal activity contributes to expansion of the anteroposterior and transverse dimensions through rib movement.

Understanding the external intercostals provides the anatomical foundation for studying inspiration, intercostal spaces, rib mechanics, thoracic wall stabilization, intercostal nerves and vessels, pump-handle and bucket-handle movements, and the mechanics of ventilation.