Learning Objective: At the end of this session, the learner should be able to identify and describe the muscles forming the thoracic wall, outline their attachments, actions, and innervation, and distinguish between primary and accessory muscles of respiration.
The thoracic wall forms the anterior, posterior, and lateral boundaries of the thoracic cavity. It plays a vital role in respiration by changing the thoracic volume during inspiration and expiration. It is primarily composed of five muscles:
- External intercostals
- Internal intercostals
- Innermost intercostals
- Subcostals
- Transversus thoracis
In addition, accessory muscles of respiration—such as the pectoralis major, pectoralis minor, serratus anterior, and scalene muscles—attach to the thoracic wall but do not form part of it directly.
Intercostal Muscles
The intercostal muscles occupy the intercostal spaces between the ribs and are arranged in three layers: external, internal, and innermost intercostals.
External Intercostals
- Number: 11 pairs
- Orientation: Fibres run inferoanteriorly, continuous with the external oblique muscle of the abdomen.
- Attachments: From the lower border of each rib to the superior border of the rib below.
- Actions:
- Stabilize ribs during normal breathing.
- Elevate ribs during forced inspiration, increasing thoracic volume.
- Innervation: Intercostal nerves (T1–T11).
Internal Intercostals
- Location: Deep to the external intercostals.
- Orientation: Fibres run inferoposteriorly, opposite to the external layer; continuous with the internal oblique muscle.
- Attachments: From the lateral edge of the costal groove to the superior surface of the rib below.
- Actions:
- Stabilize ribs during respiration.
- Depressed ribs during forced expiration, reducing thoracic volume.
- Innervation: Intercostal nerves (T1–T11).
Innermost Intercostals
- Location: Deepest layer of intercostal muscles, separated from internal intercostals by the intercostal neurovascular bundle.
- Attachments: From the medial edge of the costal groove to the superior surface of the rib below.
- Actions:
- Stabilize ribs.
- Depressed ribs during forced expiration.
- Innervation: Intercostal nerves (T1–T11).
Subcostales
- Location: Most developed in the inferior thoracic wall.
- Orientation: Fibres run in a direction similar to the innermost intercostals.
- Attachments: From the inner surface of a rib to the superior border of the second or third rib below.
- Actions:
- Depressed ribs during forced expiration.
- Innervation: Intercostal nerves.
Transversus Thoracis
- Location: On the internal surface of the anterior thoracic wall, continuous inferiorly with the transversus abdominis.
- Attachments: Arise from the posterior surface of the sternum and lower costal cartilages, inserting onto the internal surface of costal cartilages 2–6.
- Actions:
- Weakly depresses the ribs during expiration.
- Innervation: Intercostal nerves (T2–T6).
Accessory Muscles of Respiration
Although not part of the thoracic wall proper, the following muscles assist during increased respiratory demand:
- Pectoralis major and minor – Elevate the ribs when the arms are fixed.
- Serratus anterior – Assists in elevating ribs.
- Scalene muscles – Elevate the first and second ribs during deep inspiration.
