Intercostal spaces

External intercostal muscles

The external intercostal muscle (intercostales externi) consists of oblique fibers that run the length of the rib from the tubercle, posteriorly, to the costochondral junction, anteriorly. It is continuous with the aponeurotic external intercostal membrane, running from the costochondral junction to the sternum. Posteriorly, the fibers blend with the posterior fibers of the superior costotransverse ligaments. In the lower two intercostal spaces, the muscle extends to the free end of the costal cartilage and blends with the fibers of the external oblique.

The origin of the external intercostal muscle is in the inferior border of the rib above, and insertion into the superior border of the rib below, running in an inferior and forward trajectory. The external intercostals are covered with a thin, translucent membrane, that separates it from the internal intercostal muscle.

As an inspiratory muscle, contraction of the external intercostal raises the rib, increasing the anterior-posterior and transverse diameters of the thorax. Relying primarily on the diaphragm for quiet breathing, the external intercostal muscles are recruited for exertional or voluntary deep inhalation.

Internal intercostal muscles

The internal intercostal muscles (intercostales interni) consists of oblique fibers, perpendicular to the external intercostal muscle fibers, running from the angle of the rib, posteriorly, to the sternum, anteriorly. It is continuous with the internal intercostal membrane, running from the angle of the rib to the vertebral column. This aponeurotic membrane blends with the anterior fibers of the superior costotransverse ligaments. The lower internal intercostal muscles blend with the internal oblique muscle.

The origin of the internal intercostal muscle is in the costal groove of the rib, just outside the neurovascular bundle, and inserts into the above superior border of the rib below. Parasternal intercostal muscles are the thick anterior extension of the internal intercostal muscle and function as an inspiratory muscle, with the external intercostal muscles, raising the lower ribs.

As an expiratory muscle, contraction of the internal intercostal muscle depresses the rib, decreasing the anterior-posterior and transverse diameters of the thorax. Relying primarily on elastic recoil of the chest during quiet breathing, the internal intercostal muscles are recruited with exertional or forceful exhalation.

Want to learn more about the intercostal muscles? Check out our trunk wall muscle anatomy chart, complete with information on attachments, innervations and functions of every muscle in this region.

Innermost intercostal muscles

Lying between the internal intercostal muscle and the endothoracic fascia is the innermost intercostal muscle layer, which works with the internal intercostal muscles to depress the rib. The layer can be divided into three muscle groups:

• the transverse thoracic muscle, anteriorly
• the innermost intercostal muscle, laterally
• the subcostal muscle, posteriorly

Transverse thoracic muscle

Also known as the sternocostalis, the transverse thoracic muscles have an origin on the lower third of the internal aspect of the sternum. They converge and insert over the internal surface of the costal cartilages of the second to the sixth six ribs. The most superior slip of muscle is nearly vertical and they become more horizontal as they descend the thorax. The lowest slip of muscle is contiguous with the transversus abdominis muscle.  

Innermost intercostal muscles

The innermost intercostal muscles (intercostales intimi) are the lateral group. They are insignificant superiorly, becoming increasingly more prominent inferiorly. Their oblique fibers are parallel to the internal intercostal muscles and their origin and insertion are on the internal surface of the rib above and below.    

Subcostal muscle

Well-developed only in the lower, posterior thorax, subcostal muscles have their aponeurotic origins at the internal surface of the rib, near the angle, and aponeurotic insertions into the internal surface two to three ribs below. Some of the muscles may insert into the vertebral body or psoas major.

Learn about the muscles of the thoracic wall with our articles, videos, quizzes, and labeled diagrams.

Intercostal nerves

Intercostal nerves originate segmentally from the anterior rami of spinal nerves T1-T11. In addition to motor nerves to the intercostal muscles, they contain sensory afferents from the skin, intercostal muscle, ribs, pleura, and peritoneum.

The lateral cutaneous and anterior cutaneous nerves are segmental sensory nerves of the superficial soft tissues of the thorax that pierce the intercostal muscles at each level and join their corresponding intercostal nerve. The exception is the short 1st intercostal nerve, which typically has no cutaneous branches. Instead, the spinal nerve divides into a superior branch that joins the brachial plexus, and an inferior, intercostal branch that continues on the inner surface of the 1st rib.

The 2nd lateral cutaneous nerve, the intercostobrachial nerve, receives sensory input from the medial arm and axillae. The remaining intercostal nerves run between the endothoracic fascia and internal thoracic membrane posteriorly and pass between the internal and innermost intercostal muscles at the angle of the rib. Smaller collateral branches of each nerve, artery, and vein run along the superior aspect of each rib.    

Intercostal arteries

The six-upper anterior intercostal arteries originate from the internal thoracic artery, and the lower five from the musculophrenic artery, itself a branch of the internal thoracic artery. Anterior intercostal arteries form anastomoses with the posterior intercostal arteries.

The superior (supreme) intercostal artery, a branch of the costocervical trunk of the subclavian artery, branches into the first two posterior intercostal arteries. The remaining nine posterior intercostal arteries branch directly from the aorta. Both the anterior and posterior branches have perforating branches, the anterior and lateral cutaneous branches, that pierce the intercostal muscles along with the cutaneous branches of the intercostal nerves.   

Intercostal veins

The intercostal veins follow a similar distribution with the intercostal nerves and arteries. The anterior intercostal veins drain into the internal thoracic vein, adjacent to the internal thoracic artery. The first two posterior intercostal veins drain into the brachiocephalic vein, adjacent to the internal thoracic vein. The lower nine posterior intercostal veins accompany the posterior intercostal arteries, draining into the azygos system.

Intercostal lymphatics

Lymphatics following the distribution of the nerves, arteries, and veins, drain the deep tissues of the thoracic wall. The superficial tissues, including the breast, are drained by the axillary, and parasternal nodes. Lymphatics drain posteriorly into the intercostal lymph nodes, lying adjacent to the azygous and hemizygous veins.

Test your knowledge on the neurovasculature of the intercostal space with our quiz: