The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the
thorax (chest) and
pelvis, in humans and in other
vertebrates. The abdomen is the front part of the abdominal segment of the
trunk. The area occupied by the abdomen is called the
abdominal cavity. In
arthropods it is the
posterior tagma of the body; it follows the thorax or
cephalothorax.
In humans, the abdomen stretches from the thorax at the
thoracic diaphragm to the pelvis at the
pelvic brim. The pelvic brim stretches from the
lumbosacral joint (the
intervertebral disc between
L5 and
S1) to the
pubic symphysis and is the edge of the
pelvic inlet. The space above this inlet and under the thoracic diaphragm is termed the
abdominal cavity. The boundary of the abdominal cavity is the
abdominal wall in the front and the peritoneal surface at the rear.
In vertebrates, the abdomen is a large
body cavity enclosed by the abdominal muscles, at front and to the sides, and by part of the
vertebral column at the back. Lower ribs can also enclose ventral and lateral walls. The abdominal cavity is continuous with, and above, the pelvic cavity. It is attached to the
thoracic cavity by the
diaphragm. Structures such as the
aorta, inferior
vena cava and
esophagus pass through the diaphragm. Both the abdominal and pelvic cavities are lined by a serous membrane known as the
parietal peritoneum. This membrane is continuous with the
visceral peritoneum lining the organs. The abdomen in vertebrates contains a number of
organs belonging to, for instance, the
digestive system,
urinary system, and
muscular system.
Contents
The
abdominal cavity contains most organs of the
digestive system, including the
stomach, the
small intestine, and the
colon with its attached
appendix. Other digestive organs are known as the accessory digestive organs and include the
liver, its attached
gallbladder, and the
pancreas, and these communicate with the rest of the system via various ducts. The
spleen, and organs of the
urinary system including the
kidneys, and
adrenal glands also lie within the abdomen, along with many blood vessels including the
aorta and
inferior vena cava. The
urinary bladder,
uterus,
fallopian tubes, and
ovaries may be seen as either abdominal organs or as pelvic organs. Finally, the abdomen contains an extensive membrane called the
peritoneum. A fold of peritoneum may completely cover certain organs, whereas it may cover only one side of organs that usually lie closer to the abdominal wall. This is called the
retroperitoneum, and the kidneys and ureters are known as ''retroperitoneal'' organs.
Abdominal organs can be highly specialized in some animals. 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.
Sobo 1906 393.png|View of the various organs and blood-vessels in proximity with liver.
Gray1120.png|The relations of the viscera and large vessels of the abdomen, seen from behind.
Muscles
There are three layers of muscles in the
abdominal wall. They are, from the outside to the inside:
external oblique,
internal oblique, and
transverse abdominal.
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. Strength is gained by the criss-crossing of fibers, such that the
external oblique runs downward and forward, the
internal oblique upward and forward, and the transverse abdominal horizontally forward.
The
transverse abdominal muscle is flat and triangular, with its fibers running horizontally. It lies between the internal oblique and the underlying
transverse fascia. It originates from
Poupart's ligament, the inner lip of the
ilium, the
lumbar fascia and the inner surface of the
cartilages of the six lower
ribs. It inserts into the linea alba behind the
rectus abdominis.
The
rectus abdominis muscles are long and flat. The muscle is crossed by three fibrous bands called the
tendinous intersections. 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. In the region of the
groin, the
inguinal canal, is a passage through the layers. This gap is where the
testes can drop through the wall and where the fibrous cord from the
uterus in the female runs. This is also where weakness can form, and cause
inguinal hernias.
The
pyramidalis muscle is small and triangular. It is located in the lower abdomen in front of the rectus abdominis. It originates at the pubic bone and is inserted into the linea alba halfway up to the
navel.
Function

Functionally, the human abdomen is where most of the digestive tract is placed and so most of the absorption and digestion of food occurs here. The alimentary tract in the abdomen consists of the lower
esophagus, the
stomach, the
duodenum, the
jejunum,
ileum, the
cecum and the
appendix, the
ascending,
transverse and
descending colons, the
sigmoid colon and the
rectum. Other vital organs inside the abdomen include the
liver, the
kidneys, the
pancreas and the
spleen.
The
abdominal wall is split into the posterior (back), lateral (sides), and anterior (front) walls.
Movement, breathing and other functions
The abdominal muscles have different important functions. They assist as
muscles of exhalation in the breathing process during
forceful exhalation. Moreover, these muscles serve as protection for the inner organs. Furthermore, together with the back muscles they provide postural support and are important in defining the form. 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 pelvis is fixed, they can initiate the movement of the trunk in a forward motion. They also prevent
hyperextension. 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.
Posture
The transverse abdominis muscle is the deepest muscle, therefore, it cannot be touched from the outside. It can greatly affect the body's posture. The internal obliques are also deep and also affect body posture. Both of them are involved in rotation and lateral flexion of the
spine and are used to bend and support the spine from the front. The external obliques are more superficial and they are also involved in rotation and lateral flexion of the spine. Also they stabilize the spine when upright. The rectus abdominis muscle is not the most superficial abdominal muscle. The tendonous sheath extending from the external obliques cover the rectus abdominis. The rectus abdominis is the muscle that very fit people develop into the 6-pack ab look. Although, it should really be a 10 pack as there are 5 vertical sections on each side. The 2 bottom sections are just above the pubic bone and usually not visible, hence, the 6 pack abs. The rectus abdominals' function is to bend one's back forward (flexion). The main work of the abdominal muscles is to bend the spine forward when contracting concentrically.
Society and culture
Social and cultural perceptions of the outward appearance of the abdomen has varying significance around the world. Depending on the type of society,
excess weight can be perceived as an indicator of wealth and prestige due to excess food, or as a sign of poor health due to lack of exercise. In many cultures, bare abdomens are distinctly sexualized and perceived similarly to
breast cleavage.
Exercise
Being key elements of spinal support, and contributors to good posture, it is important to properly exercise the abdominal muscles together with the back muscles because when these are weak or overly tight they can suffer painful spasms and
injuries. When properly exercised, abdominal muscles contribute to improved posture and balance, reduce the likelihood of
back pain episodes, reduce the severity of back pain, protect against injury by responding efficiently to stresses, help avoid some back surgeries, and help with the healing of back problems, or after spine surgery. When strengthened, the abdominal muscles provide flexibility as well. The abdominal muscles can be worked by practicing disciplines of general body strength such as
Pilates,
yoga,
T'ai chi, and
jogging. There are also specific routines which target each of these muscles.
Clinical significance
Abdominal obesity is a condition where
abdominal fat or visceral fat, has built up excessively between the abdominal organs. This is associated with a higher risk of
heart disease,
asthma and
type 2 diabetes.
Abdominal trauma is an injury to the abdomen and can involve damage to the abdominal organs. There is an associated risk of
severe blood loss and
infection.
Injury to the lower chest can cause injuries to the spleen and liver.
A scaphoid abdomen is when the abdomen is sucked inwards. In a newborn, it may represent a
diaphragmatic hernia. In general, it is indicative of
malnutrition.
Disease
Many
gastrointestinal diseases affect the abdominal organs. These include
stomach disease,
liver disease,
pancreatic disease,
gallbladder and
bile duct disease; intestinal diseases include
enteritis,
coeliac disease,
diverticulitis, and
IBS.
Examination
Different
medical procedures can be used to examine the organs of the gastrointestinal tract. These include
endoscopy,
colonoscopy,
sigmoidoscopy,
enteroscopy,
oesophagogastroduodenoscopy and
virtual colonoscopy. There are also a number of
medical imaging techniques that can be used. Surface landmarks are important in the
examination of the abdomen.
Surface landmarks

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. At about its midpoint sits the umbilicus or
navel. The
rectus abdominis on each side of the linea alba stands out in muscular people. The outline of these muscles is interrupted by three or more transverse depressions indicating the
tendinous intersections. There is usually one about the xiphoid process, one at the navel, and one in between. It is the combination of the linea alba and the tendinous intersections which form the abdominal "six-pack" sought after by many people.
The upper lateral limit of the abdomen is the subcostal margin (at or near the ''
subcostal plane'') formed by the cartilages of the
false ribs (8, 9, 10) joining one another. The lower lateral limit is the anterior crest of the
ilium and
Poupart's ligament, which runs from the anterior superior spine of the ilium to the spine of the
pubis. These lower limits are marked by visible grooves. 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.
One method by which the location of the abdominal contents can be appreciated is to draw three horizontal and two vertical lines.
=Horizontal lines
=

* The highest of the former is the
transpyloric line of C. Addison, which is situated halfway between the
suprasternal notch and the top of the pubic symphysis, and often cuts the pyloric opening of the stomach an inch to the right of the mid-line. The
hilum of each
kidney is a little below it, while its left end approximately touches the lower limit of the
spleen. It corresponds to the first lumbar vertebra behind.
* The second line is the ''
subcostal line'', drawn from the lowest point of the
subcostal arch (
tenth rib). It corresponds to the upper part of the third lumbar vertebra, and it is an inch or so above the umbilicus. It indicates roughly the
transverse colon, the lower ends of the kidneys, and the upper limit of the transverse (3rd) part of the
duodenum.
* The third line is called the ''
intertubercular line'', and runs across between the two rough
tubercles, which can be felt on the outer lip of the crest of the ilium about from the anterior superior spine. This line corresponds to the body of the fifth lumbar vertebra, and passes through or just above the
ileo-caecal valve, where the
small intestine joins the
large intestine.
=Vertical lines
=
The two vertical or mid-Poupart lines are drawn from the point midway between the anterior superior spine and the pubic symphysis on each side, vertically upward to the costal margin.
* The right one is the most valuable, as the
ileo-caecal valve is situated where it cuts the intertubercular line. The orifice of the
appendix lies an inch lower, at
McBurney's point. In its upper part, the vertical line meets the transpyloric line at the lower margin of the ribs, usually the ninth, and here the
gallbladder is situated.
* The left mid-Poupart line corresponds in its upper three-quarters to the inner edge of the
descending colon.
The right subcostal margin corresponds to the lower limit of the
liver, while the right nipple is about half an inch above its upper limit.
Quadrants and regions

The abdomen can be divided into quadrants or regions to describe the location of an organ or structure. Classically, quadrants are described as the left upper, left lower, right upper, and right lower. Quadrants are also often used in describing the site of an abdominal pain.
The abdomen can also be divided into nine regions.
These terms stem from "hypo" meaning "below" and "epi" means "above", while "chondron" means "cartilage" (in this case, the cartilage of the rib) and "gaster" means stomach. The reversal of "left" and "right" is intentional, because the anatomical designations reflect
the patient's own right and left.)
The "right iliac fossa" (RIF) is a common site of pain and tenderness in patients who have
appendicitis. The fossa is named for the underlying
iliac fossa of the
hip bone, and thus is somewhat imprecise. Most of the anatomical structures that will produce pain and tenderness in this region are not in fact in the concavity of the ileum. However, the term is in common usage.
Other animals
thumb|In the worker ant, the abdomen consists of the thorax_and_the_
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thorax_(insect_anatomy)">thorax_and_the_[[metasoma,_itself_divided_into_the_narrow_[[petiole_(insect_anatomy)">petiole_and_bulbous_[[gaster_(insect_anatomy).html" style="text-decoration: none;"class="mw-redirect" title="metasoma">thorax (insect anatomy)">thorax and the
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In [[arthropod]]s the abdomen is built up of a series of upper plates known as [[tergite]]s and lower plates known as [[sternite]]s, the whole being held together by a tough yet stretchable membrane.
The abdomen contains the insect's digestive tract and reproductive organs, it consists of eleven segments in most orders of insects though the eleventh segment is absent in the adult of most higher orders. The number of these segments does vary from species to species with the number of segments visible reduced to only seven in the common
honeybee. In the
Collembola (Springtails) the abdomen has only six segments.
The abdomen is sometimes highly modified. In
Apocrita (bees, ants and wasps), the first segment of the abdomen is fused to the
thorax and is called the
propodeum. In
ants the second segment forms the narrow
petiole. Some ants have an additional
postpetiole segment, and the remaining segments form the bulbous
gaster.
The petiole and gaster (abdominal segments 2 and onward) are collectively called the
metasoma.
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.
In
arachnids (spiders, scorpions and relatives), the term "abdomen" is used interchangeably with "
opisthosoma" ("hind body"), which is the body section posterior to that bearing the legs and head (the prosoma or
cephalothorax).
See also
*
Abdominal fat
References
External links
*
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Category:Animal anatomy
Category:Human anatomy