Figure 2-1
The thoracic cage.

Respiratory muscles act to expand the size and volume of the thoracic cage. This expansion creates negative pressure (vacuum) within the thorax and lungs. The vacuum attracts air from outside the thoracic cavity through the nose, mouth and trachea to fill the lungs. Muscle relaxation, gravity and lung elasticity function to return the thorax and lungs to their resting position.

The number of muscles active in respiration depends on the intensity of the breathing effort. During normal or quiet respiration, the intercostal muscles and the diaphragm are the active respiratory muscles. During forced respiration, the pectoralis major, pectoralis minor and latissimus dorsi join the diaphragm and intercostal muscles.

The intercostal muscles occupy the intercostal spaces between the ribs to complete the wall for the thorax. Intercostal muscles are arranged in two layers: external intercostal muscles and internal intercostal muscles. (Figure 2-1)