The rib cage (thoracic cage) is a bony structure that protects the heart and lungs and provides the framework for breathing. It is a dynamic component of the body, covered by diverse muscle groups. These muscles serve two purposes: they facilitate the rhythmic motion of respiration, and they anchor the upper limbs and core musculature. Understanding these muscular layers shows how the body balances internal protection with external flexibility.
Muscles Directly Between the Ribs
The intercostal muscles are the most direct muscles covering the ribs, filling the spaces between adjacent ribs in three distinct layers. The external intercostal muscles form the most superficial layer, with fibers running downward and forward. These muscles are active during inhalation, contracting to pull the ribs upward and outward. This action increases the volume of the chest cavity, drawing air into the lungs.
Beneath the external layer are the internal intercostal muscles, whose fibers run diagonally in the opposite direction. While quiet exhalation is typically a passive process, the internal intercostals become active during forceful exhalation, such as when coughing or exercising. Their contraction pulls the ribs downward and inward, reducing the chest cavity’s volume to expel air.
The deepest layer is the innermost intercostal muscle group, which runs parallel to the internal intercostals. This group functions as a synergistic partner in depressing the ribs during active expiration. A space separating this deepest layer from the internal intercostals contains the neurovascular bundle, including the intercostal nerve and blood vessels. Collectively, these three layers reinforce the chest wall and maintain its structural integrity against pressure changes during breathing.
External Muscles for Movement and Stability
The rib cage is also covered by external muscles that connect the trunk to the shoulder girdle, facilitating upper body movement. The Pectoralis Major muscle is a large, fan-shaped muscle on the upper chest that attaches to the sternum and the cartilages of the first six ribs. Its primary function is moving the arm across the body, such as in pushing or hugging motions. It also aids in drawing the ribs up during deep inhalation.
Beneath the Pectoralis Major is the Pectoralis Minor, which originates on the third through fifth ribs. This smaller muscle does not move the arm directly. Instead, it stabilizes the shoulder blade (scapula) by pulling it forward and downward against the rib cage. This stabilizing action provides a firm base for the arm muscles during overhead movements.
The Serratus Anterior muscle originates from the first eight or nine ribs and wraps around to attach to the inner surface of the scapula. This muscle is responsible for protraction, which is the forward movement of the shoulder blade, such as when throwing a punch. It also prevents the scapula from lifting off the chest wall during arm movements, an action known as “winging.” The Latissimus Dorsi, a broad muscle of the back, attaches to the lower three to four ribs and is responsible for powerful pulling motions, including extending and adducting the arm.
Abdominal Muscles Attached to the Lower Ribs
The lower margin of the rib cage serves as a major attachment point for the muscles of the anterior and lateral abdominal wall. These muscles provide core stability and trunk movement. The External Oblique muscle, the largest and most superficial of the side abdominal muscles, originates from the lower eight ribs. Its fibers run downward and forward, allowing for trunk rotation and lateral bending.
Deep to the external oblique is the Internal Oblique muscle, which attaches to the lower three to six ribs. It works in concert with the external oblique on the opposite side to twist the trunk. Both oblique muscles assist in pulling the ribs down during forceful expiration, contributing to the expulsion of air.
The Rectus Abdominis, the long, vertical muscle pair often called the “six-pack,” inserts onto the cartilage of the fifth, sixth, and seventh ribs. Its contraction flexes the trunk, pulling the rib cage closer to the pelvis. It also increases intra-abdominal pressure for activities like coughing or heavy lifting. These abdominal muscles collectively protect the internal organs and link the movements of the torso and lower body to the chest.