The deep (intrinsic) chest muscles form the muscular framework of the intercostal spaces. Their primary role is to drive the biomechanics of respiration through coordinated rib movements and maintain the structural stability of the thoracic cage.
LayersThree layers are identified: external, internal, and innermost intercostal muscles.
RespirationExternal muscles assist with inspiration, while internal and innermost muscles facilitate expiration.
TopographyThe neurovascular bundle lies medial (internal) to the costal groove.
External Intercostal Muscles
The external intercostal muscles (musculi intercostales externi) form the superficial layer of the intrinsic chest wall muscles.
Structural and Topographical Features:
Origin and Fiber Direction: The muscles originate from the tubercles of the ribs. Their fibers run anteriorly and inferiorly, occupying the external surface of the intercostal spaces down to the costochondral junctions.
Membrane: In the anterior thoracic wall (between the costal cartilages), muscle fascicles are absent. Here, they are replaced by the external intercostal membrane (membrana intercostalis externa).
Function: They are essential muscles of inspiration — upon contraction, they elevate the ribs, increasing the volume of the thoracic cavity.
Innervation: Supplied by the intercostal nerves (nn. intercostales), segments Th1–Th11.
Blood Supply: Supplied by the posterior intercostal arteries (aa. intercostales posteriores), as well as the internal thoracic artery (a. thoracica interna) and musculophrenic artery (a. musculophrenica).
Internal Intercostal Muscles
The internal intercostal muscles (musculi intercostales interni) lie deep to the externals and act as antagonists during respiration.
Key Characteristics:
Origin and Fiber Direction: They originate from the angles of the ribs and run anteriorly and superiorly, reaching the sternum. Crucially, their fibers run perpendicularly (crosswise) relative to the fibers of the external intercostal muscles.
Membrane: In the posterior regions (from the costal angles to the vertebral column), the muscular tissue transitions into the internal intercostal membrane (membrana intercostalis interna).
Function: They participate in expiration, depressing the ribs. Additionally, they play an important role in stabilizing the intercostal spaces, preventing them from bulging or retracting during changes in intrathoracic pressure.
Innervation and Blood Supply: Innervated by the same intercostal nerves. Supplied by the anterior and posterior branches of the intercostal arteries (aa. intercostales).
Innermost Intercostal Muscles
The innermost intercostal muscles (musculi intercostales intimi) represent the deepest layer of the thoracic wall muscular apparatus.
Topography and Functions:
Location: They lie directly adjacent to the internal surface of the thoracic wall. The muscle bundles are positioned medial to the costal groove (sulcus costae).
Neurovascular Bundle: The intercostal vessels and nerve travel precisely between the internal and innermost intercostal muscles, within the projection of the sulcus costae. Together, they form a single complex — the neurovascular bundle.
Function: Like the internal intercostal muscles, they act synergistically in expiration and contribute to rigid rib stabilization.
Innervation and Blood Supply: Completely identical to those of the internal intercostal muscles.
Mnemonic
External muscles work for INSPIRATION (elevate ribs), while internals work for EXPIRATION (depress ribs). The direction of their fibers forms an "X" shape (running crosswise to each other).
Frequently asked questions
Which muscles belong to the deep (intrinsic) chest muscles?
The deep (intrinsic) chest muscles comprise six groups:
External intercostals (mm. intercostales externi).
Internal intercostals (mm. intercostales interni).
Innermost intercostals (mm. intercostales intimi).
Subcostals (mm. subcostales).
Transversus thoracis (m. transversus thoracis).
Levatores costarum — long and short (mm. levatores costarum longi et breves).
In what order from top to bottom are the elements of the intercostal neurovascular bundle arranged in the costal groove?
The elements of the intercostal neurovascular bundle in the costal groove are arranged from top to bottom as: vein, artery, nerve.
Intercostal vein (v. intercostalis) — occupies the highest position.
Intercostal artery (a. intercostalis) — located in the middle.
Intercostal nerve (n. intercostalis) — occupies the lowest position.
The mnemonic VAN is used to remember this order.
What are the exact points of origin and insertion for the internal intercostal muscles?
The internal intercostal muscles (mm. intercostales interni) originate from the angles of the ribs and run anteriorly and superiorly, reaching the sternum.
They run crosswise relative to the external intercostals. Posteriorly, these muscles continue as the internal intercostal membrane (membrana intercostalis interna). Sources do not list specific isolated origin and insertion points for each individual rib, noting only the general course of the muscle fibers.
Which deep chest muscles participate in expiration?
Two groups of deep chest muscles participate in expiration.
Internal intercostals (mm. intercostales interni) — depress the ribs and stabilize the intercostal spaces.
Innermost intercostals (mm. intercostales intimi) — also participate in expiration and stabilize the rib.
The functions of the other deep chest muscles (subcostals, transversus thoracis, and levatores costarum) regarding respiration are not described in the provided materials.
Which deep chest muscles drive inspiration?
Only the external intercostal muscles participate in inspiration (elevating the ribs).
Where is the intercostal neurovascular bundle located?
It runs deep to the costal groove (sulcus costae), forming the neurovascular bundle.
What replaces the intercostal muscles where they terminate?
Anteriorly, the external muscles transition into the external intercostal membrane, while posteriorly, the internal muscles continue as the internal intercostal membrane.
Go deeper
Formation and composition of the thoracic wall neurovascular bundle
Structure and topography of the external intercostal membrane
Structure and topography of the internal intercostal membrane
Anastomoses of the intercostal arteries (anterior and posterior branches)
Biomechanics of intercostal space stabilization during respiration