The thoracic wall forms the anterior, posterior and lateral boundaries of the thorax. It is comprised of the five muscles, which act to change the volume of the thoracic cavity during respiration.
- External, internal and innermost intercostals
- Subcostals
- Transversus thoracis
There are additional muscles that do not form the thoracic wall, but attach to it. These are termed accessory muscles of respiration and include the pectoralis major, minor, serratus anterior and the scalene muscles.
In this article, we shall look at the anatomy of the muscles of the thoracic wall – their attachments, actions and innervation.
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Intercostals
The intercostal muscles lie in the intercostal spaces between ribs. They are organised into three layers.
External Intercostals
There are 11 pairs of external intercostal muscles. They run inferoanteriorly from the rib above to the rib below, and are continuous with the external oblique of the abdomen.
- Attachments: Originates at the lower border of each rib, inserting onto the superior border of the rib below.
- Actions: Stablises the position of the ribs during normal respiration. Elevates the ribs and increases the thoracic volume during forced inspiration.
- Innervation: Intercostal nerves (T1-T11).
Internal Intercostals
The internal intercostals lie deep to the external intercostals. They extend from the rib above to the one below, but in an opposite direction to the external intercostals (inferoposteriorly).
They are continuous with the internal oblique muscle of the abdominal wall.
- Attachments: Originates from the lateral edge of the costal groove and inserts onto the superior surface of the rib below.
- Actions: Stablises the position of the ribs during normal respiration. Depresses the ribs and decreases the thoracic volume during forced expiration.
- Innervation: Intercostal nerves (T1-T11).
Innermost Intercostals
The innermost intercostals are the deepest of the intercostal muscles and are similar in structure to the internal intercostals.
They are separated from the internal intercostals by the intercostal neurovascular bundle. They are found in the most lateral portion of the intercostal spaces.
- Attachments: Originates from the medial edge of the costal groove and inserts into the superior surface of the rib below.
- Actions: Stablises the position of the ribs during normal respiration. Depresses the ribs and decreases the thoracic volume during forced expiration.
- Innervation: Intercostal nerves (T1-T11)
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Transversus Thoracis
The transversus thoracis is located on the internal surface of the thoracic wall and is continuous with the transversus abdominis inferiorly.
- Attachments: Arises from the posterior surface of the inferior sternum and costal cartilage of the lower ribs. The fibres ascend and attach onto the internal surface of costal cartilages 2-6.
- Actions: Weak depressor of the ribs during expiration.
- Innervation: Intercostal nerves (T2-T6).
Fig 2
View of the internal aspect of the thoracic wall. The internal intercostal and transverse thoracis muscles are visible.
Subcostales
The subcostal muscles are most developed in the inferior portion of the thoracic wall. The direction of the fibres is similar to that of the innermost intercostal muscles.
- Attachments: Originates from the inferior surface of the lower ribs. Attaches to the superior border of the second or third rib below.
- Actions: Stablises the position of the ribs during normal respiration. Depresses the ribs and decreases the thoracic volume during forced expiration.
- Innervation: Intercostal nerves.