Origin: Pubic crest and symphysis.
Insertion: Costal cartilages of the fifth through seventh ribs and xiphoid process of the sternum.
Direction of Fibers: Vertical.
Action: Flexes the vertebral column by approximating the thorax and pelvis anteriorly. With the pelvis fixed, the thorax will move toward the pelvis; with the thorax fixed, the pelvis will move toward the thorax.
Weakness: Weakness of this muscle results in decreased ability to flex the vertebral column. In the supine position, the ability to tilt the pelvis posteriorly or approximate the thorax toward the pelvis is decreased, making it difficult to raise the head and upper trunk. For anterior neck flexors to raise the head from a supine position, the anterior abdominal muscles (particularly the rectus ab-dominis) must fix the thorax. With marked weakness of the abdominal muscles, an individual may not be able to raise the head even though the neck flexors are strong. In the erect position, weakness of this muscle permits an anterior pelvic tilt and a lordotic posture (i.e., increased anterior convexity of the lumbar spine).
Above the arcuate line (1), the aponeurosis of the internal oblique (b) divides. Its anterior lamina fuses with the aponeurosis of the external oblique (a) to form the ventral layer of the rectus sheath. Its posterior lamina fuses with the aponeurosis of the transversus abdominis (c) to form the dorsal layer of the rectus sheath.
Below the arcuate line (2), the aponeuroses of all three muscles fuse to form the ventral layer of the rectus sheath, and the transversalis fascia forms the dorsal layer. (See also p. 197.)
EXTERNAL OBLIQUE, ANTERIOR FIBERS
Origin: External surfaces of ribs five through eight in-terdigitating with the serratus anterior.
Insertion: Into a broad, flat aponeurosis, terminating in the linea alba, which is a tendinous raphe that extends from the xiphoid.
Direction of Fibers: Obliquely downward and medially, with the uppermost fibers more medial.
Action: Acting bilaterally, the anterior fibers flex the vertebral column (approximating the thorax and pelvis anteriorly), support and compress the abdominal viscera, depress the thorax and assist in respiration. Acting unilaterally with the anterior fibers of the internal oblique on the opposite side, the anterior fibers of the external oblique rotate the vertebral column, bringing the thorax forward (when the pelvis is fixed), or the pelvis backward (when the thorax is fixed). For example, with the pelvis fixed, the right external oblique rotates the thorax counter-clockwise, and the left external oblique rotates the thorax clockwise.
Nerves to anterior and lateral fibers: (T5, 6), T7-11, T-12
EXTERNAL OBLIQUE, LATERAL FIBERS
Origin: External surface of the ninth rib, interdigitating with the serratus anterior; and external surfaces of the 10th through 12th ribs, interdigitating with the latis-simus dorsi.
Insertion: As the inguinal ligament, into the anterosu-perior spine and pubic tubercle and into the external Up of the anterior h of the iliac crest.
Direction of Fibers: Fibers extend obliquely downward and medially, but more downward than the anterior fibers.
Action: Acting bilaterally, the lateral fibers of the external oblique flex the vertebral column with a major influence on the lumbar spine, tilting the pelvis posteriorly. (See also action in relation to posture, p. 71.) Acting unilaterally with the lateral fibers of the internal oblique on the same side, these fibers of the external oblique laterally flex the vertebral column, approximating the thorax and iliac crest. These external oblique fibers also act with the internal oblique on the opposite side to rotate the vertebral column. The external oblique, in its action on the thorax, is comparable to the sternocleidomastoid in its action on the head.
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