Topographical regions of the chest wall
For precise localization of pathological processes and surgical approaches, the anterolateral chest wall is conventionally divided into four main regions:
- Sternal region (regio sternalis). Located most medially, running directly along the sternum. Topographically, it corresponds precisely to the anterior median line of the body.
- Pectoral region (regio pectoralis). A broad area projected over the location of the pectoralis major muscle. The mammary gland is also situated here.
- Axillary region (regio axillaris). Topographically corresponds to the lower division of the shoulder girdle. Includes the axillary fossa. This zone has distinct muscular boundaries formed by the pectoralis major, latissimus dorsi, and serratus anterior muscles.
- Inframammary region (regio inframammaria, or regio hypochondriaca anterior). The lowest segment of the anterolateral wall. It extends inferiorly down to the costal arches, where it transitions anatomically directly into the hypochondriac regions.
Serratus anterior muscle
This muscle (m. serratus anterior) is a crucial element for the fixation and movement of the scapula. Its maximum development and largest muscle mass are observed in the projection zone of the inferior angle of the scapula.
- Points of origin: the muscle originates via 8–9 pronounced muscular digitations from the outer surface of the upper ribs, from ribs I through IX.
- Point of insertion: the fibers run posteriorly and attach to the anterior surface of the scapula — along its medial border and at the inferior angle.
- Biomechanics (function): the primary task of the muscle is to pull the scapula anteriorly and laterally. This causes lateral and upward rotation of the scapula, which is a mandatory condition for elevating the upper limb above the horizontal plane. In addition, the muscle closely presses and fixes the scapula to the thorax. Notably, the lower bundles act as a powerful stabilizer and accessory muscle during inspiration.
- Innervation: supplied by the long thoracic nerve (n. thoracicus longus), formed from segments C5–C7.
- Blood supply: provided by branches of the thoracodorsal (a. thoracodorsalis), lateral thoracic (a. thoracica lateralis), and intercostal arteries (aa. intercostales).
Subcostal muscles
The subcostal muscles (mm. subcostales) belong to the intrinsic deep musculature of the thoracic cage and are located on the inner surface of its framework.
- Localization: they lie predominantly in the posterolateral parts of the chest wall, attaching to the inner surface of the lower ribs. A distinctive feature of their bundles is that they do not attach to the adjacent rib, but instead skip 1–2 ribs inferiorly.
- Biomechanics (function): these are muscles of expiration. Upon contraction, they actively pull the ribs inferiorly and medially, reducing the volume of the thoracic cavity.
- Innervation: intercostal nerves (nn. intercostales), corresponding to segments Th7–Th12.
- Blood supply: nourishment is supplied by the posterior intercostal arteries (aa. intercostales posteriores).
Briefly: answers to topical questions
Which muscles belong to the superficial chest muscles?
The superficial chest muscles include four muscles that embryologically originate from the upper limb.
- Pectoralis major muscle (m. pectoralis major)
- Pectoralis minor muscle (m. pectoralis minor)
- Subclavius muscle (m. subclavius)
- Serratus anterior muscle (m. serratus anterior)
Which muscles belong to the deep (intrinsic) muscles of the chest?
The deep, or intrinsic, chest muscles include six muscle groups.
- External intercostal muscles (mm. intercostales externi)
- Internal intercostal muscles (mm. intercostales interni)
- Innermost intercostal muscles (mm. intercostales intimi)
- Subcostal muscles (mm. subcostales)
- Transversus thoracis muscle (m. transversus thoracis)
- Levatores costarum — long and short (mm. levatores costarum longi et breves)
Where does the pectoralis major muscle originate and insert?
The pectoralis major muscle (m. pectoralis major) originates from the clavicle, sternum, ribs, and the sheath of the rectus abdominis muscle, and inserts into the humerus.
- Origin: medial half of the clavicle (pars clavicularis), anterior surface of the sternum and cartilages of ribs II–VII (pars sternocostalis), as well as the anterior wall of the rectus sheath (pars abdominalis).
- Insertion: crest of the greater tubercle of the humerus. The tendon forms a double U-shaped fold, with the upper bundles lying distal to the lower ones.
Which fascial layers are distinguished in the chest region?
In the chest region, the superficial pectoral fascia and the parietal thoracic fascia are distinguished.
- Pectoral fascia (fascia pectoralis) — covers the anterior chest wall. It features a superficial layer (lamina superficialis), a deep layer (lamina profunda), and the clavipectoral fascia (fascia clavipectoralis).
- Thoracic fascia (parietal) (fascia thoracica) — a thin layer covering the intercostal muscles and ribs externally across the entire chest wall.
Where does the pectoralis minor muscle originate and insert?
The pectoralis minor muscle (m. pectoralis minor) belongs to the superficial chest muscles. It originates on the anterior surface of the thorax and inserts into the scapula.
- Origin: anterior ends and cartilages of ribs II through V.
- Insertion: coracoid process of the scapula.
Where is the transversus thoracis muscle located and what functions does it perform?
The transversus thoracis muscle (m. transversus thoracis) originates from the inner surface of the lower part of the sternum, the xiphoid process, and the cartilages of ribs III–VI. It inserts into the inner surfaces of the cartilages of ribs II–VI.
- Functions: acts as a weak expiratory muscle, pulling the rib cartilages posteriorly and inferiorly; tenses the anterior wall of the thoracic cage.
Class and exam questions
How many anatomical regions is the anterolateral chest wall divided into?
In topographical anatomy, exactly 4 regions are distinguished: sternal, pectoral, axillary, and inframammary.
Which muscle enables a person to raise their arm above the shoulder girdle (horizontal) level?
This movement is provided by the serratus anterior muscle. Upon contraction, it rotates the scapula with its inferior angle laterally and upward, carrying along the glenoid cavity and the entire upper limb.
Where exactly are the subcostal muscles located and what is their specific feature?
They are located on the inner surface of the posterolateral parts of the lower ribs. Their main feature is that their fiber bundles cross not to the adjacent rib, but skip one or two ribs inferiorly.
What else to study on this topic
- Anatomy and boundaries of the axillary fossa
- Intrinsic chest fasciae: superficial, pectoral, and endothoracic
- Topography and structure of the mammary gland
- Remaining superficial and deep chest muscles (pectoralis major/minor, intercostals)
- Course and branching of the long thoracic nerve
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