Internal Intercostals

What Are the Internal Intercostals

The internal intercostals constitute the intermediate layer of the intercostal muscles, occupying the intercostal spaces between the ribs alongside the external and innermost intercostals. As part of this group, their main role is aiding respiration.

Anatomy

Location and Attachments

Origin	The lateral side of the costal groove of the 1st to 11th ribs
Insertion	The superior border of the 2nd to 12th ribs

Origin

There are 22 internal intercostal muscles in the human body, 11 each on the left and right side of the rib cage. Each of these muscles originates from the rib superior to it, specifically, the inner surface of their costal groove.

Insertion

From their point of origin, the muscle fibers run laterally and obliquely toward the back of the rib cage to their insertion point into the superior border of the corresponding rib below.

So, these muscles cover the area from the sternum at the front of your chest, all the way to the back of the ribcage. Then, the muscle fibers blend into a thin aponeurosis to form the internal (posterior) intercostal membrane.

Relations With Surrounding Muscles and Structures

The external intercostals lie superficial to these muscles, while the innermost intercostals lie beneath them. The internal and external intercostals are separated by a thin fascia.

The two lowermost internal intercostals form a continuous musculature with the internal oblique muscle, the abdominal muscle that inserts into the 10th to 12th ribs.

The internal intercostals, along with the innermost intercostals, surround the passage through which the neurovascular bundle (the intercostal veins, arteries, and nerves) passes.

Function

Action

Holding the ribs steady during respiration and depressing the rib cage during forceful exhalation

In Respiration

These muscles originate at the front of the rib cage and are inserted at the back, running at a 90° angle with the external intercostal muscle fibers. This arrangement enables the two layers to function antagonistically to compress and expand the thoracic cavity alternately during breathing.

In Vocalization

When the internal intercostals contract, they pull the ribs downward to reduce the volume of the thoracic cavity so the air in the lungs rushes out. This function makes them one of the vital respiratory muscles for vocalization. When speaking or singing, the air needs to rush out through the mouth and nose for us to make a sound. In fact, the pressure of this escaping air is key to deciding how loud one’s voice will be. 

Supporting the Thoracic Wall

Another important function of these muscles, along with the rest of the intercostal muscles, is to keep the rib cage and thoracic wall stable. It prevents the rib cage from collapsing or the structures in the thoracic cavity, like the heart and lungs, from bulging out.

Antagonists

The external intercostal muscles are antagonistic to the internal intercostal muscles, as they work on the thoracic cavity in opposite directions.

Innervation

Nerve

Corresponding branches of the intercostal nerves

Blood Supply

Artery

Anterior and posterior intercostal arteries

The anterior intercostal arteries that supply the 1st to 6th intercostal spaces are branches of the internal thoracic artery, which arises from the subclavian artery. On the other hand, the rest of the anterior intercostal arteries of the 7th to 9th intercostal spaces rise from the internal thoracic artery’s terminal branch, the musculophrenic artery.

The 1st and 2nd intercostal spaces also receive vasculature from the posterior intercostal artery, which branches from the superior intercostal artery, originating from the subclavian artery. Posterior intercostal arteries supply the 3rd to 11th intercostal spaces directly from the thoracic segment of the descending aorta.