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Thoracic Wall Muscles: Intercostal, Serratus & Levatores

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Thoracic Wall Muscles

MUSC. Chest

External Intercostal Muscles

INTERCOSTAL Externs: Located in the deep chest wall extending from the dorsal to the ventral aspect.

  • O (Origin): Inferior edge of the rib above, external to the costal groove.
  • I (Insertion): Superior edge of the rib below.
  • Innervation: Intercostal nerves.
  • Action: Elevate the ribs; increase inspiration.
  • Relations: Covered by the levatores costarum muscles and parts of the serratus posterior and by other superficial back and chest muscles (for example, latissimus dorsi, pectoralis major); associated with rectus and the external and internal oblique abdominal muscles. Overlies the internal intercostals.

Internal Intercostal Muscles

INTERCOSTAL Interns: These muscles do not reach the spine posteriorly; posteriorly their position is occupied by their fascia.

  • O (Origin): Superior edge of the rib below.
  • I (Insertion): Inferior edge of the rib above.
  • Innervation: Intercostal nerves.
  • Action: Approximate the ribs; act as expiratory muscles.
  • Relations: Covered anteriorly by the external intercostals near the sternum (triangularis sterni area). Continuous with or related to the subcostal muscles; internally related to internal subcostal and other deep thoracic muscles.

Serratus Posterior Superior

Serratus dorsalis SB: Located in the deep superior region of the dorsal trunk.

  • O (Origin): Broad aponeurosis from the spinous processes of approximately C7 to T2 (last cervical and first thoracic vertebrae).
  • I (Insertion): Fleshy digitations to the 2nd to 5th ribs.
  • Innervation: Intercostal nerves (ventral rami) T1–T4 (commonly described as intercostal nerves 1 to 4).
  • Action: Elevate the ribs; assist in inspiration.
  • Relations: Deep to rhomboids and trapezius near the scapular angle; overlapped by splenius and deep back muscles (longissimus, iliocostalis) and adjacent to the external intercostals.

Serratus Posterior Inferior

Serratus dorsalis INF: Located in the deep inferior region of the dorsal trunk.

  • O (Origin): Thoracolumbar fascia in the lower thoracic and upper lumbar region (from the region of the last thoracic vertebra and first lumbar vertebra).
  • I (Insertion): Four large, variable digitations to the inferior borders of the last four ribs.
  • Innervation: Intercostal nerves (commonly described as T9–T11).
  • Action: Depress the ribs; involved in expiration and may help prevent excessive ascent of the diaphragm.
  • Relations: Covered superficially by latissimus dorsi; lies over iliocostalis and longissimus and has slight contact with the abdominal obliques.

Levatores Costarum Muscles

Levatores costarum muscles: Small muscles in the thoracic dorsal column area.

  • O (Origin): Tips of the transverse processes from about C7 to T11 (processes costotransverse from the 7th cervical vertebra to the 11th thoracic vertebra).
  • I (Insertion): To the rib immediately below (short fibers) or to the second rib below (long fibers).
  • Innervation: Anterior rami (branches of the intercostal nerves).
  • Action: Elevate the ribs; act as accessory muscles of inspiration.
  • Relations: Deep to the external intercostals; overlain by longissimus and iliocostalis. Related to intertransversarii, rotatores, and spinalis muscles.

External Intercostal (Long Fibers)

INTERCOSTAL Externs LONG:

  • O (Origin): Inferior edge of the rib above.
  • I (Insertion): Superior edge of the second rib below.
  • Innervation: Intercostal nerves.
  • Action: Elevate the ribs; contribute to inspiration.

Subcostal Muscles

Subcostal:

  • O (Origin): Internal surface near the superior edge of lower ribs.
  • I (Insertion): Inferior edge of ribs two or three levels above.
  • Innervation: Intercostal nerves.
  • Action: Draw adjacent ribs together; expiratory muscles.
  • Relations: Covered by the internal intercostals; related to the diaphragm.

Transversus Thoracis (Triangularis Sterni)

TRIANGULAR sternum: Also known as the transversus thoracis or triangularis sterni.

  • O (Origin): Internal surface of the anterior thoracic wall: internal aspect of the body of the sternum and the superior surface of the xiphoid process.
  • I (Insertion): Via separate bundles to the inferior edges of the 2nd to 6th costal cartilages.
  • Innervation: Anterior rami of thoracic nerves (intercostal nerves from approximately T2 to T6).
  • Action: Contraction acts on the thoracic wall and assists in expiration.
  • Relations: Related to the internal intercostals and adjacent to the sternal portion of the diaphragm.

Notes: The above content preserves the original structure and details while correcting spelling, grammar, and capitalization. Terminology has been standardized (for example, "Externs" → "External Intercostal"). All original elements—origins, insertions, innervation, actions, and relations—are retained and clarified for readability and SEO.

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