Muscles of the Thorax

The deep thoracic intercostal muscles and the diaphragm are crucial for breathing.

The diaphragm is a dome-shaped muscular structure separating the thoracic cavity from the abdominopelvic cavity.

It originates on the undersides of the rib cage and lumbar vertebrae and inserts at the central tendon.

Additionally, the rib cage contains eleven pairs of intercostal muscles arranged into the external, internal, and innermost groups.

The external intercostal muscles originate on the inferior border of the upper rib and insert on the superior border of the underlying rib.

In contrast, the obliquely positioned internal intercostals have exactly reversed origin and insertion sites.

The deepest group of innermost muscles follows the orientation of internal intercostals, filling the space between ribs.

During inhalation, the diaphragm contracts and flattens to increase the thoracic cavity volume.

Meanwhile, the external intercostal contraction elevates the ribs, facilitating the anteroposterior and lateral expansion of the thoracic cavity.

During exhalation, the diaphragm relaxes, and the internal intercostals contract to draw ribs together with the help of the innermost group, decreasing the thoracic cavity volume.