Innermost Intercostal Muscles

The innermost intercostal muscles form the deepest layer of the intercostal muscle group within the thoracic wall. They are often less continuous and less robust than the external and internal intercostals but share similar attachments and functions. These muscles assist in forced expiration by depressing the ribs and help maintain thoracic wall stability. They also play a crucial anatomical role in forming the plane that houses the intercostal neurovascular bundle.

Location

The innermost intercostal muscles are located deep to the internal intercostal muscles and superficial to the parietal pleura. They are present in the middle portion of the intercostal spaces, particularly in the lateral thoracic region. They do not typically span the full anterior-posterior extent of the rib cage like the external and internal intercostals.

Origin and Insertion

• Origin: Internal surface of the inferior border of the rib above
• Insertion: Internal surface of the superior border of the rib below

These muscles are often segmented and incomplete compared to the other intercostals. Their fibers run in the same direction as the internal intercostals: inferolaterally (downward and backward).

Fiber Direction

Fibers of the innermost intercostal muscles run parallel to those of the internal intercostals—inferolaterally—creating continuity of function and structural reinforcement within the intercostal space.

Extent and Distribution

The innermost intercostals are typically present only in the lateral portion of the intercostal spaces. Anteriorly, they are replaced by the transversus thoracis muscles, and posteriorly by the subcostal muscles, both of which are considered part of the same functional layer.

Components of the Deep Intercostal Layer

The innermost intercostal plane is composed of three muscle groups:

• Innermost intercostal muscles: Found laterally within intercostal spaces
• Subcostal muscles: Found posteriorly, spanning multiple ribs
• Transversus thoracis: Found anteriorly on the inner surface of the sternum

Layer Relationships

From superficial to deep, the arrangement of structures within the intercostal space is as follows:

1. External intercostal muscle
2. Internal intercostal muscle
3. Intercostal neurovascular bundle (vein, artery, nerve) in the costal groove
4. Innermost intercostal muscle
5. Parietal pleura

The intercostal vessels and nerves lie between the internal and innermost intercostals, making this an important anatomical reference for surgical access.

Function

• Assist in forced expiration by depressing the ribs
• Contribute to thoracic wall rigidity and structural integrity
• Stabilize intercostal spaces during respiration and movement

Their role is minor during quiet breathing but becomes more significant during activities that require active expiration such as coughing, singing, or exertion.

Blood Supply

• Arterial:
  • Anterior intercostal arteries (branches of internal thoracic and musculophrenic arteries)
  • Posterior intercostal arteries (branches of thoracic aorta)
• Venous:
  • Anterior and posterior intercostal veins → internal thoracic vein and azygos system

Innervation

• Intercostal nerves (T1–T11), derived from the anterior rami of thoracic spinal nerves

These nerves travel in the neurovascular plane between the internal and innermost intercostal layers.

Embryological Origin

The innermost intercostal muscles develop from thoracic myotomes of the paraxial mesoderm. Like the other intercostal layers, their segmental organization reflects the arrangement of ribs and nerves.

Distinguishing Features

• Smaller and less defined than external or internal intercostals
• Limited to lateral parts of the intercostal spaces
• Form part of the deepest muscular layer along with subcostals and transversus thoracis

Comparative Table: Intercostal Muscle Layers

Clinical Relevance

• Surgical importance: During thoracic surgeries, awareness of the neurovascular bundle between the internal and innermost intercostals is crucial to prevent vascular and nerve injury
• Thoracentesis: A needle is inserted above the upper border of the rib to avoid the VAN bundle between the internal and innermost layers