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The linea semilunaris (also semilunar line or Spigelian line) is described by Adriaan van den Spiegel (°1578 †1625) described the Linea Semilunaris as the line forming and marking the transition from muscle to aponeurosis in the transversus abdominis muscle of the abdomen. It needs to be distinguished from the lateral border of the rectus ...
The linea alba (Latin for: white line) is a strong fibrous midline structure [1] of the anterior abdominal wall [2] situated between the two recti abdominis muscles (one on either side). The umbilicus (navel) is a defect in the linea alba through which foetal umbilical vessels pass before birth. [ 1 ]
In the middle line, it interlaces with the aponeurosis of the opposite muscle, forming the linea alba, which extends from the xiphoid process to the pubic symphysis. That portion of the aponeurosis which extends between the anterior superior iliac spine and the pubic tubercle is a thick band, folded inward, and continuous below with the fascia ...
The rectus abdominis is a very long flat muscle, which extends along the whole length of the front of the abdomen, and is separated from its fellow of the opposite side by the linea alba. Tendinous intersections (intersectiones tendineae) further subdivide each rectus abdominis muscle into a series of smaller muscle bellies.
The external oblique functions to pull the chest downwards and compress the abdominal cavity, which increases the intra-abdominal pressure as in a Valsalva maneuver.It also performs ipsilateral (same side) side-bending and contralateral (opposite side) rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa.
The muscle fibers run from these points superomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th through 12th ribs and the linea alba. In males, the cremaster muscle is also attached to the internal oblique.
Since the tendons of the internal oblique and transversus abdominis only reach as high as the costal margin, it follows that above this level the sheath of the rectus is deficient behind, the muscle resting directly on the cartilages of the ribs, and being covered only by the tendons of the external obliques.
Throughout the rest of its extent the aponeurosis passes horizontally to the middle line, and is inserted into the linea alba; its upper three-fourths lie behind the rectus muscle and blend with the posterior lamella of the aponeurosis of the internal oblique; its lower fourth is in front of the rectus abdominis.