The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach[citation needed]) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates.

[1] In humans, the abdomen stretches from the thorax at the thoracic diaphragm to the pelvis at the pelvic brim.

The abdominal cavity contains most organs of the digestive system, including the stomach, the small intestine, and the colon with its attached appendix.

[3] The first three layers extend between the vertebral column, the lower ribs, the iliac crest and pubis of the hip.

All of their fibers merge towards the midline and surround the rectus abdominis in a sheath before joining up on the opposite side at the linea alba.

It originates from the inguinal ligament, costal cartilages 7-12, the iliac crest and thoracolumbar fascia.

The rectus abdominis is enclosed in a thick sheath, formed as described above, by fibers from each of the three muscles of the lateral abdominal wall.

They originate at the pubis bone, run up the abdomen on either side of the linea alba, and insert into the cartilages of the fifth, sixth, and seventh ribs.

This gap is where the testes can drop through the wall and where the fibrous cord from the uterus in the female runs.

Other vital organs inside the abdomen include the liver, the kidneys, the pancreas and the spleen.

When the glottis is closed and the thorax and pelvis are fixed, they are integral in the cough, urination, defecation, childbirth, vomit, and singing functions.

When the thorax is fixed, they can pull up the pelvis and finally, they can bend the vertebral column sideways and assist in the trunk's rotation.

The rectus abdominis is the muscle that very fit people develop into "six-pack" abs, though there are five vertical sections on each side.

[4] Social and cultural perceptions of the outward appearance of the abdomen has varying significance around the world.

In many cultures, bare abdomens are distinctly sexualized and perceived similarly to breast cleavage.

In the mid-line, a slight furrow extends from the xiphoid process above to the pubic symphysis below, representing the linea alba in the abdominal wall.

It is the combination of the linea alba and the tendinous intersections which form the abdominal "six-pack" sought after by many people.

Just above the pubic spines on either side are the external abdominal rings, which are openings in the muscular wall of the abdomen through which the spermatic cord emerges in the male, and through which an inguinal hernia may rupture.

The "right iliac fossa" (RIF) is a common site of pain and tenderness in patients who have appendicitis.

Most of the anatomical structures that will produce pain and tenderness in this region are not in fact in the concavity of the ileum.

For example, the stomach of ruminants, (a suborder of mammals that includes cattle and sheep), is divided into four chambers – rumen, reticulum, omasum and abomasum.

In Apocrita (bees, ants and wasps), the first segment of the abdomen is fused to the thorax and is called the propodeum.

Unlike other arthropods, insects possess no legs on the abdomen in adult form, though the Protura do have rudimentary leg-like appendages on the first three abdominal segments, and Archaeognatha possess small, articulated "styli" which are sometimes considered to be rudimentary appendages.

Many larval insects including the Lepidoptera and the Symphyta (sawflies) have fleshy appendages called prolegs on their abdominal segments (as well as their more familiar thoracic legs), which allow them to grip onto the edges of plant leaves as they walk around.