External Intercostals

What Are the External Intercostals

The external intercostals are the most superficial intercostal muscles that fill the intercostal spaces in the ribcage. Along with the internal and innermost intercostals, the primary function of the external intercostals is to assist in respiration.

Anatomy

Location and Attachments

Origin	The lower borders of the 1st to 11th ribs
Insertion	The upper borders of the 2nd to 12th ribs

Origin

Each of the 11 paired muscles originates from the sharp inferior border of each rib, except the 12th. 

They extend from the tubercle of the ribs at the back to the point where they articulate with the costal cartilage (costochondral junctions). There are a few exceptions. The upper 2-3 external intercostals do not cover the entire rib. Contrarily, the 2 lowest muscles extend until their respective costal cartilages end.

At the back, The muscle fibers course inferolaterally, while they course medially at the front of the ribcage.

Insertion

As is typical for intercostal muscles, each external intercostal inserts into the superior border of the rib below the one it originated from.

Relations With Surrounding Muscles and Structures

As evident from the name, they cover the internal intercostals and innermost intercostals that lie deep, with only a thin facia separating these muscles from each other. The external intercostals have the serratus anterior, the pectoralis major and minor, and the superior portion of the rectus abdominis lying superior to it.

In the back, the muscle fibers blend with the superior costotransverse ligaments. From their point of insertion at the front, near the costal cartilage, the muscles turn into thin membranes. These membranes are called the external (or anterior) intercostal membranes, and they cover the rest of the respective intercostal spaces between the costochondral junctions and the sternum.

The lower external intercostals fuse with the external oblique muscle lying laterally at the front of the abdominal wall.

Function

Action

Supporting the ribcage and elevating it to increase its volume during forced inspiration

Helping with Respiration

All the intercostal muscles work together to help with respiration. When the external intercostal muscles contract, all the ribs elevate, causing the ribcage to expand, allowing air to enter the lungs. This happens because the ribs are connected with each other by the 11 external intercostals. These muscles work in coordination with the diaphragm and remain active until their receptors signal that the lungs are full. The internal intercostals take over from this point.

Though the external intercostal muscles play a vital role during normal breathing, they are more active during forced or deep inhalation, such as when you take a deep breath before singing or when there is a lack of oxygen.

Supporting the Thoracic Wall

Along with the internal and innermost intercostals, these muscles play a vital role in maintaining the shape and integrity of the thoracic wall. It prevents the thoracic cavity from collapsing and the organs and structures here from bulging out. 

Antagonists

Since the internal intercostal muscles work to move the ribs in the opposite direction of the external intercostals, the two are considered antagonists.

Innervation

Nerve

Intercostal nerves

Each muscle’s innervation comes from the intercostal nerves from the corresponding intercostal spaces. These nerves branch from the anterior rami of the T1 to T11 spinal nerves and are part of the somatic nervous system.

Blood Supply

Artery

Anterior and posterior intercostal arteries

The vascular supply to these muscles is quite extensive. The anterior intercostal arteries that supply the muscles of the upper 6 intercostal spaces directly arise from the internal thoracic artery. The arteries supplying the 7th to 9th intercostal spaces arise from the musculophrenic artery, a branch of the internal thoracic artery.

As for the posterior intercostal arteries, those of the first 2 intercostal spaces are branches of the superior intercostal artery that branches from the costocervical trunk (a branch of the subclavian artery). The muscles in the remaining 9 intercostal spaces receive vasculature from posterior intercostal arteries that are branches of the descending thoracic aorta.