Thoracic Cage

Muscles of the Thorax

We will look at five muscles that act on the thoracic cage:

  1. External intercostal
  2. Internal intercostal
  3. Innermost intercostal
  4. Subcostals
  5. Transversus Thoracis

These muscles are important to change the volume of the thoracic cavity during respiration.

Intercostals

There are slight variations between the different intercostal spaces, but typically each space contains three muscles, comparable to those of the abdominal wall, and an associated neurovascular bundle. The muscles are:

The fibres pass downwards and forwards.
 
Origin: lower border of the rib
 
Attachment: superior border of the rib below.
 

Innervation: Intercostal nerves (T1-T11).

Action: Elevate the ribs (E for external = E for elevate). This increases thoracic volume.
 
This lies deep to external intercostal. Runs downwards and backwards from the sternum to the angles of the ribs where it becomes the  posterior intercostal membrane.
 
Origin: lateral edge of the costal groove
 
Attachment: superior border of the lower rib
 
Innervation: Intercostal nerves (T1-T11).
 
Action: Interchondral part (anterior) elevates the ribs. Interosseus part (posterior) depresses the rib.

This muscle is only incompletely separated from the internal intercostal muscle by the neurovascular bundle. The fibres of this sheet cross more than one intercostal space and it may be incomplete.

Origin: medial edge of the costal groove.

Attachment: superior surface of the rib below.

Innervation: Intercostal nerves (T1-T11)

Action: Interchondral part (anterior) elevates the ribs. Interosseus part (posterior) depresses the rib.

Subcostals

The subcostal muscles are thin strips of muscle found at the inferior portion of the thoracic wall, on the internal surface of the ribs. 

Origin: inferior surface of the lower ribs, near the angle of the rib.

Attachment:  superior border of the rib 2 or 3 below.

Innervation: Intercostal nerves

Action: Interchondral part (anterior) elevates the ribs. Interosseus part (posterior) depresses the rib.

Transversus Thoracis

This thin muscle is continous with the transversus abdominis. It is located on the internal aspect of the anterior thoracic wall.

Origin: inferoposterior surface of the sternum and xiphoid process.

Attachment: internal surface of costal cartilages 2-6.

Innervation: Intercostal nerves (T2-T6).

Action: Depresses the rib (weak).

Clinical Features

 
  • Local irritation of the intercostal nerves may give rise to referred pain on the anterior chest or abdomen in the region of the peripheral termination of the nerves. This could happen in conditions such as Pott’s disease of the thoracic vertebrae (tuberculosis).
  • Intercostal nerve block can produce effective anaesthesia to the thoracic muscles. Local anaesthesia is infiltrated into the intercostal space around the intercostal nerve trunk and its collateral branch.
  • During conventional posterolateral thoracotomy (e.g. for a pulmonary lobectomy) an incision is made along the line of the 5th or 6th rib; the periosteum over a segment of the rib is elevated, thus protecting the neurovascular bundle, and the rib is excised.  Access to the lung or mediastinum is then gained though the intercostal space, which can be opened out considerably owing to the elasticity of the thoracic cage. 
  • Localised abscess from the vertebral column can track around the thorax along the course of the neurovascular bundle at the exit sites of cutaneous branches of the intercostal nerves:
  • lateral to erector spinae (sacrospinalis),
      1. in the midaxillary line
      2. lateral to the sternum.
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