Subcostal Muscles

Subcostal muscles are a small group of thoracic wall muscles located on the internal posterior aspect of the rib cage. Though often overlooked due to their thin and variable presence, they contribute to the deepest muscular layer of the thoracic wall alongside the innermost intercostal and transversus thoracis muscles. Subcostal muscles assist in forced expiration by depressing the ribs and provide minor reinforcement to the posterior thoracic wall.

Location

These muscles are found on the inner surface of the posterior thoracic wall, primarily near the angles of the lower ribs. They span one or more intercostal spaces and are best developed in the lower thoracic region (typically T10–T12).

Subcostal muscles lie deep to the internal intercostal muscles and posterior to the parietal pleura. Their presence is often asymmetric and inconsistent across individuals.

Origin and Insertion

• Origin: Internal surface of the lower ribs near their angles
• Insertion: Internal surface of the rib 1 or 2 levels below, near the superior border

The muscles are oriented in a similar direction to the internal intercostal muscles, with fibers running inferoposteriorly (downward and backward). They often span more than one intercostal space, which differentiates them structurally from the shorter intercostals.

Fiber Direction

Fibers run in the same direction as the internal intercostal muscles—obliquely downward and backward. This arrangement allows them to function similarly during expiration.

Layer Relationships

Subcostal muscles form part of the innermost layer of thoracic wall musculature, specifically in the posterior region. From superficial to deep in the posterior thoracic wall, the layering is:

1. External intercostal muscle (posterolateral)
2. Internal intercostal muscle
3. Subcostal muscle (posterior portion of innermost layer)
4. Parietal pleura

They are located internal to the intercostal neurovascular bundle and are continuous with the innermost intercostals laterally and with the transversus thoracis anteriorly.

Function

• Forced expiration: Depress the ribs, reducing thoracic volume during active exhalation
• Thoracic stability: Reinforce the posterior aspect of the intercostal spaces
• Support rib mechanics: Maintain spacing and resist bulging during respiration

Although their respiratory contribution is minor, subcostal muscles act synergistically with internal and innermost intercostals, especially during vigorous expiratory efforts such as coughing or forceful speaking.

Blood Supply

• Arterial supply:
  • Posterior intercostal arteries (branches of thoracic aorta)
  • Collateral branches from the anterior intercostal arteries (less commonly)
• Venous drainage:
  • Posterior intercostal veins → azygos system

Innervation

• Intercostal nerves (T10–T12), derived from the anterior rami of thoracic spinal nerves

These nerves also supply the overlying internal and innermost intercostal muscles, as well as skin and pleura in the associated dermatomes.

Embryological Origin

Like other thoracic wall muscles, subcostal muscles develop from the thoracic myotomes of the paraxial mesoderm. Their segmental organization and nerve supply reflect their somite-based origin.

Anatomical Variation

• Subcostals are often absent or poorly developed in the upper thorax
• The number of muscle slips varies between individuals, and they may span one or two ribs
• They are more prominent and consistent in the lower thoracic region

Relationship to Other Muscles

• Internal intercostals: Superficial to the subcostals; shorter in length and span a single intercostal space
• Innermost intercostals: Found laterally; together with subcostals and transversus thoracis form the innermost muscular plane
• Transversus thoracis: Anterior counterpart in the same muscle layer

Comparative Table: Deep Thoracic Wall Muscles

Clinical Relevance

• Surgical landmark: Subcostals are rarely visualized during routine procedures but may be encountered during posterior thoracic surgeries
• Thoracic wall strength: Weakness or absence does not cause dysfunction but may slightly reduce structural reinforcement posteriorly