External intercostal muscles

Origin and insertion

The external intercostal muscles consist of 11 pairs of muscles. Every external intercostal originates from the sharp inferior costal border of one rib. The fibers of each muscle course inferomedially and insert along the outer lip of the superior border of the immediate rib below.

Relations

The external intercostal muscles are located superficially to the internal intercostal muscles, separated from them by a thin fascia. Superficial to the external intercostal muscles lie the pectoralis major and minor, serratus anterior and the upper part of the rectus abdominis muscle.

Each intercostal muscle occupies its intercostal space starting from the tubercles of ribs posteriorly, to the costochondral junctions anteriorly. The exceptions are the upper two or three muscles that do not reach the costochondral junction, and the lower two that reach even the free ends of the costal cartilages. The most posterior fibers of each muscle fuse with the superior costotransverse ligaments. On the other hand, the most anterior fibers blend with the external intercostal membrane, which fills the remainder of the corresponding intercostal space from the costochondral junction to the sternum. The external intercostals found in the lower thoracic levels blend with the external abdominal oblique muscles in the anterolateral abdominal wall. 

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Innervation

The external intercostal muscles receive innervation from intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves.

Blood supply

The external intercostal muscles have an extensive vascular supply. They are supplied by the muscular branches of the anterior and posterior intercostal arteries. 

• The anterior intercostal arteries supplying the muscles of the upper 6 intercostal spaces arise from the internal thoracic artery directly, whereas the lower 3 arise from its branch; the musculophrenic artery 
• The first 2 posterior intercostal arteries arise from the supreme intercostal artery, a branch of the costocervical trunk. The remaining 9 arise from the posterior aspect of the thoracic aorta.

Functions

Being one of the accessory respiratory muscles, the external intercostals elevate ribs during forced inspiration. This increases the transverse and anteroposterior diameter of the lungs, which in turn decreases the intrapleural pressure. This process expands the lungs and facilitates the entry of air into them.

Additionally, it is found that the static tension of all three groups of intercostal muscles has a role in the mechanical support of the chest wall during various movements involving trunk.