Muscles of the Trunk
The muscles of the thoracic wall are primarily involved in breathing. The muscles of the abdominal wall protect and support the organs within the abdominal cavity.
Muscles of the Thoracic Wall
External intercostal muscles occur between the ribs, they originate on a superior rib and insert on an inferior rib. These muscles elevate the rib cage during the inspiration phase of breathing.
The diaphragm is a domehaped muscle that, as you know, separates the thoracic cavity from the abdominal cavity. Contraction of the diaphragm also assists inspiration.
Internal intercostal muscles originate on an inferior rib and insert on a superior rib. These muscles depress the rib cage and contract only during a forced expiration. Normal expiration does not require muscular action.
Muscles of the Abdominal Wall
The abdominal wall has no bony reinforcement (Fig. 1.12). The wall is strengthened by four pairs of muscles that run at angles to one another.The external and internal obliques and the transversus abdominis occur laterally, but the fasciae of these muscle pairs meet at the midline of the body, forming a tendinous area called the linea alba. The rectus abdominis is a superficial medial pair of muscles. All of the muscle pairs of the abdominal wall compress the abdominal cavity and support and protect the organs within the abdominal cavity.
External and internal obliques occur on a slant and are at right angles to one another between the lower ribs and the pelvic girdle. The external obliques are superior to the internal obliques. These muscles also aid trunk rotation and lateral flexion.
Transversus abdominis, deep to the obliques,extends horizontally across the abdomen. The obliques and the transversus abdominis are synergistic muscles.
Rectus abdominis has a straplike appearance but takes its name from the fact that it runs straight (rectus means straight) up from the pubic bones to the ribs and sternum. These muscles also help flex and rotate the lumbar portion of the vertebral column.
