15.11: Muscles of the Thorax
The thorax muscles are central to the body's respiration and provide essential support and movement for the upper body. They are intricately designed to facilitate the complex breathing process while also contributing to the structural integrity and mobility of the chest and upper limbs.
The diaphragm is at the core of thoracic musculature, the primary muscle involved in breathing. This expansive, dome-shaped muscle marks the division between the thoracic and abdominal cavities. It originates beneath the rib cage and lumbar vertebrae and attaches to the central tendon. When it contracts, the diaphragm moves downward and flattens, increasing the volume of the thoracic cavity and drawing air into the lungs. During exhalation, it relaxes and returns to its dome shape, helping expel air from the lungs.
The rib cage has eleven pairs of intercostal muscles arranged into three groups: the external, the internal, and the innermost groups. The external intercostal muscles originate on the lower edge of the upper rib and attach to the upper edge of the underlying rib. They are most active during inhalation, where they contract to elevate the ribs and expand the chest cavity. On the other hand, the obliquely positioned internal intercostals have the opposite origin and insertion sites. They primarily assist in forced exhalation, pulling the ribs downward and inward to decrease the thoracic cavity's volume. The innermost intercostals, often considered a part of the internal layer, mirror the action of the internal intercostals and are similarly active during exhalation, helping compress the ribcage.
Additionally, the thorax houses muscles like the pectoralis major, pectoralis minor, serratus anterior, and subclavius, which facilitate shoulder and upper limb movements and stabilize the shoulder girdle. These muscles work with the respiratory muscles to support a range of upper body movements and postural adjustments.
