INTERNAL OBLIQUE, LOWER ANTERIOR FIBERS
Origin: Lateral h of inguinal ligament and short attachment on iliac crest near the anterosuperior spine.
Insertion: With the transverse abdominis into crest of the pubis, medial part of the pectineal line and into the linea alba by means of an aponeurosis.
Direction of Fibers: Transversely across the lower abdomen.
Action: The lower anterior fibers compress and support the lower abdominal viscera in conjunction with the transversus abdominis.
INTERNAL OBLIQUE, UPPER ANTERIOR FIBERS
Origin: Anterior '/3 of intermediate line of the iliac crest.
Insertion: Linea alba by means of an aponeurosis.
Direction of Fibers: Obliquely medially and upward.
Action: Acting bilaterally, the upper 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 in conjunction with the anterior fibers of the external oblique on the opposite side, the upper anterior fibers of the internal oblique rotate the vertebral column, bringing the thorax backward (when the pelvis is fixed), or the pelvis forward (when the thorax is fixed). For example, the right internal oblique rotates the thorax clockwise, and the left internal oblique rotates the thorax counterclockwise on a fixed pelvis.
INTERNAL OBLIQUE, LATERAL FIBERS
Origin: Middle h of intermediate line of the iliac crest and the thoracolumbar fascia.
Insertion: Inferior borders of the 10th through 12th ribs and the linea alba by means of an aponeurosis.
Direction of Fibers: Obliquely upward and medially, but more upward than the anterior fibers.
Action: Acting bilaterally, the lateral fibers flex the vertebral column (approximating the thorax and pelvis anteriorly) and depress the thorax. Acting unilaterally with the lateral fibers of the external oblique on the same side, these fibers of the internal oblique laterally flex the vertebral column, approximating the thorax and pelvis. These fibers also act with the external oblique on the opposite side to rotate the vertebral column.
Nerves to Anterior and Lateral Fibers: T7, 8,19-11
LI, iliohypogastric and ilioinguinal, ventral rami.
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