The thoracic muscles make up the walls and floor of the thoracic cavity - the space within the rib cage that protects the heart and lungs and plays a central role in breathing. 

These muscles work together to move the ribs and diaphragm, helping to change the volume and pressure of the thoracic cavity during respiration.

The thoracic musculature can be divided into two major groups:

• Muscles of the thoracic wall which move and stabilize the ribs.

• The diaphragm, which forms the floor of the thoracic cavity and is the primary muscle responsible for breathing.

External Intercostals

The external intercostals stabilize the ribs and elevate them during forced inspiration, increasing chest volume. It is the most superficial layer, continuous with the external oblique of the abdomen.

• Attachments: From the lower edge of one rib to the upper edge of the rib below.

• Innervation: Intercostal nerves (T1–T11).

Internal Intercostals

The internal intercostals stabilize the ribs and depress them during forced expiration, decreasing chest volume. It's located deep to the external intercostals, running in the opposite direction, and continuous with the internal oblique of the abdomen.

• Attachments: From the inside of a rib’s costal groove to the rib below.

• Innervation: Intercostal nerves (T1–T11).

Innermost Intercostals

The innermost intercostals are similar to internal intercostals – they stabilize the ribs and depress them during forced expiration. It is the deepest layer, separated from the internal intercostals by the intercostal nerves and blood vessels.

• Attachments: From the inner surface of one rib to the rib below.

• Innervation: Intercostal nerves (T1–T11).

Transversus Thoracis

The transversus thoracis is a thin, flat muscle located on the inner surface of the anterior (front) thoracic wall. It forms part of the deep layer of thoracic musculature and is considered the thoracic counterpart of the transversus abdominis, with which it is continuous inferiorly. 

It weakly depresses the ribs during expiration, which helps reduce thoracic cavity volume, assisting forced exhalation.

• Origin: The posterior surface of the lower sternum, including the xiphoid process.

• Insertion: The inner surfaces of the costal cartilages of ribs 2 through 6.

• Innervation: Intercostal nerves (T2–T6).

Subcostals

The subcostal muscles are a small group of thoracic wall muscles located mainly in the lower (inferior) part of the rib cage. They stabilize the rib cage during breathing and assist the innermost intercostals in reinforcing the posterior thoracic wall.

Their fiber direction is similar to the innermost intercostals, running obliquely downward, and they span more than one rib, which helps distinguish them from the other intercostal muscles. 

• Origin: The inner (underside) surface of a lower rib.

• Insertion: The upper border of the second or third rib below the origin.

• Innervation: Intercostal nerves.

The Diaphragm

The diaphragm is a broad, dome-shaped sheet of muscle that forms the floor of the thoracic cavity and separates it from the abdominal cavity. It is the primary muscle of respiration, meaning it does most of the work in breathing.

The diaphragm is located at the base of the rib cage. The right dome sits slightly higher than the left because of the liver beneath it.

It has three peripheral attachments and a central tendon:

• Lumbar vertebrae and arcuate ligaments

• Costal cartilages of ribs 7–10 and ribs 11–12

• Xiphoid process of the sternum

The muscle fibers from these attachments converge into a central tendon, which connects to the fibrous pericardium (the membrane around the heart).

The diaphragm also has right and left crura, which are tendinous structures that anchor it to the lumbar vertebrae:

• Right crus: Arises from vertebrae L1–L3 and their discs; surrounds the esophagus to prevent acid reflux.

• Left crus: Arises from vertebrae L1–L2 and their discs.

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