The oblique and rectus muscles lie within the body wall, between the spinous processes of vertebrae and the ventral midline (Figures 11-3 and 11-12  and Table 11-9). The oblique muscles compress underlying structures or rotate the vertebral column, depending on whether one or both sides are contracting. The rectus muscles are important flexors of the vertebral column, acting in opposition to the erector spinae. The oblique and rectus muscles share embryological origins. We can divide them into cervical, thoracic, and abdominal groups.

The oblique group includes the scalene muscles of the neck (Figure 11-11c) and the intercostal and transversus muscles of the thorax (Figure 11-12a, b). The scalene muscles (anterior, middle, and posterior) elevate the first two ribs and assist in flexion of the neck. In the thorax, the oblique muscles extend between the ribs, and the external intercostal muscles cover the internal intercostal muscles. Both groups of intercostal muscles aid in respiratory movements of the ribs. A small transversus thoracis muscle crosses the inner surface of the rib cage and is covered by the pleura, the serous membrane that lines the pleural cavities.    The sternum occupies the place where we might expect thoracic rectus muscles to be.

The same basic pattern of musculature extends unbroken across the abdominopelvic surface (Figure 11-12 a, c). Here the muscles are called the external abdominal oblique, internal abdominal oblique, transversus abdominis, and rectus abdominis muscles. The rectus abdominis muscle inserts at the xiphoid process and originates near the pubic symphysis. This muscle is longitudinally divided by the linea alba (white line), a median collagenous partition (Figure 11-3a ). The rectus abdominis muscle is separated into segments by transverse bands of collagen fibers called tendinous inscriptions. Each segment contains muscle fibers that extend longitudinally, originating and inserting on the tendinous inscriptions.

The Diaphragm

The term diaphragm refers to any muscular sheet that forms a wall. When used without a modifier, however, diaphragm, or diaphragmatic muscle, specifies the muscular partition that separates the abdominopelvic and thoracic cavities (Figure 11-12b). We include this muscle here because it develops in association with the other muscles of the chest wall. The diaphragm is a major muscle of respiration.