The area beneath the rib cage forms the boundary between the upper abdomen and the lower chest cavity. This region contains several internal organs and a large sheet of muscle. Understanding the architecture of this space clarifies the physical sensations experienced there. The primary muscle occupying this transitional zone is responsible for the body’s most continuous action.
The Diaphragm: Location and Structure
The diaphragm is the primary muscle located beneath the ribs, a large, thin, musculotendinous structure that forms a physical partition between the chest cavity and the abdominal cavity. Its resting position is a distinct double-dome shape, with the right dome typically sitting slightly higher than the left due to the liver’s placement underneath. The muscle is firmly anchored to the skeletal framework of the lower torso.
The diaphragm’s muscular fibers radiate outward from a strong, flat central tendon. These fibers originate from three distinct peripheral points of attachment. Anteriorly, they connect to the posterior surface of the xiphoid process, the small cartilaginous section at the bottom of the sternum. Laterally, the muscle is secured to the inner surfaces of the lower six ribs and their costal cartilages.
Posteriorly, the diaphragm attaches to the lumbar vertebrae of the spine through tendinous bands known as the crura. This extensive ring of attachments ensures that the muscle is positioned to function as the floor of the thoracic cavity and the roof of the abdominal cavity.
The Mechanics of Breathing
The diaphragm is the principal muscle of quiet respiration, performing the bulk of the work required for normal breathing. Inhalation is an active process initiated when the muscle receives a signal from the brain via the phrenic nerve. Upon contraction, the dome-shaped diaphragm flattens and moves downward toward the abdominal cavity, pushing the abdominal contents inferiorly.
This downward movement increases the vertical dimension of the thoracic cavity, enlarging the space surrounding the lungs. Because the lungs adhere to the inner wall of the chest, they are forced to expand with the cavity, which lowers the pressure inside the lungs compared to the outside air. This pressure gradient draws air into the lungs.
Exhalation during quiet breathing is a mostly passive event that relies on the natural elasticity of the lungs and the chest wall. As the diaphragm relaxes, it returns to its upward, dome-like resting position. This relaxation, along with the elastic recoil of the lung tissue, decreases the volume of the chest cavity, forcing the air back out of the lungs.
Supporting Musculature of the Rib Cage
While the diaphragm is the main muscle of breathing, other muscles are integrated into the rib cage to assist in the process, especially during physical activity or forced breathing. The intercostal muscles are a set of small muscles located in the spaces between the ribs, arranged in two principal layers: the external and internal intercostals.
The external intercostals work with the diaphragm during inhalation by pulling the ribs upward and outward, which further expands the chest cavity volume. The internal intercostals serve the opposite role during forced expiration, helping to pull the ribs downward to rapidly decrease the chest volume and expel air. These muscles also provide structural stability to the rib cage.
Another muscle in the area is the transversus thoracis, a thin sheet of muscle found on the internal surface of the anterior chest wall. This muscle is considered an accessory muscle of expiration, helping to depress the ribs, particularly during forceful breathing. Together, these muscles work in a coordinated effort to manage the constant, rhythmic changes in volume necessary to sustain respiration.