Abdomen
The abdomen answers to several names. In vertebrates it is also called the belly or the stomach, and in arthropods the region goes by the name metasoma. A person and an ant both have one, yet the two are built very differently. What exactly is the abdomen, and what holds it in place? Which organs does it shelter, and what do its muscles do besides keep everything in? How do doctors map it, and what happens when it fails? And why does an insect's version look so unlike ours?
In a human, the top of the abdomen sits at the thoracic diaphragm and the bottom at the pelvic brim. The brim runs from the disc between the L5 and S1 vertebrae to the pubic symphysis, and it marks the edge of the pelvic inlet. The space above that inlet and beneath the diaphragm is the abdominal cavity. Its boundary is the abdominal wall at the front and the peritoneal surface at the rear.
Across vertebrates generally, the abdominal muscles enclose the cavity at the front and sides, and part of the vertebral column closes off the back. The lower ribs can also help wall in the sides and front. Below, the cavity runs on without a break into the pelvic cavity. Above, the diaphragm joins it to the thoracic cavity.
The aorta, the inferior vena cava and the esophagus all pass through that diaphragm. A thin serous membrane called the parietal peritoneum lines both the abdominal and the pelvic cavities. It continues seamlessly into the visceral peritoneum, which wraps the organs themselves. How far that wrapping reaches turns out to matter, because some organs sit behind it entirely.
The stomach, the small intestine and the colon with its attached appendix form the core of what the cavity holds. Three further digestive organs, the liver with its gallbladder and the pancreas, are called accessory organs. They reach the rest of the system through a network of ducts.
The spleen, the kidneys and the adrenal glands also lie here, along with many blood vessels. The urinary bladder, uterus, fallopian tubes and ovaries are a matter of definition. Anatomists may count them as abdominal organs or as pelvic ones.
The peritoneum treats its tenants unevenly. A fold of it can cover an organ completely, or it can cover only the side of an organ that lies near the abdominal wall. That second arrangement is called the retroperitoneum, and the kidneys and ureters are retroperitoneal organs.
For the digestive tract, the cavity is where most digestion and absorption of food takes place. Food passes from the lower esophagus to the stomach, then through the duodenum, jejunum and ileum. It continues into the cecum and appendix, then up the ascending colon, across the transverse colon and down the descending colon. The sigmoid colon and the rectum finish the route.
The wall guarding all of this is divided into posterior, lateral and anterior parts, and its front is built from layered muscle.
Three muscles lie stacked in the wall, and from the outside in they are the external oblique, the internal oblique and the transverse abdominal. Their fibers run in different directions. The external oblique slopes downward and forward, the internal oblique upward and forward, and the transverse abdominal straight across. That criss-crossing is what gives the wall its strength.
The first three layers stretch between the vertebral column, the lower ribs, the iliac crest and the pubis of the hip. Their fibers converge toward the midline and wrap the rectus abdominis in a sheath. On the far side they join their counterparts at the linea alba.
The transverse abdominal is flat and triangular, lying between the internal oblique and the transverse fascia. It begins at the inguinal ligament, costal cartilages 7-12, the iliac crest and the thoracolumbar fascia. It ends at the conjoint tendon, the xiphoid process, the linea alba and the pubic crest.
The rectus abdominis is long and flat, and three fibrous bands called tendinous intersections cross it. It starts at the pubis and climbs either side of the linea alba to the cartilages of the fifth, sixth and seventh ribs. Very fit people develop this muscle into the famous six-pack. It is not actually the outermost muscle, since a sheath from the external obliques covers it. The two lowest sections sit just above the pubic bone and are usually not visible.
The look of the abdomen carries different meanings around the world. Where food is abundant, excess weight can signal wealth and prestige. Elsewhere it is read as poor health from lack of exercise. In many cultures a bare abdomen is sexualized, in much the way breast cleavage is.
A small triangular muscle, the pyramidalis, sits in the lower abdomen in front of the rectus. It runs from the pubic bone to the linea alba, about halfway up to the navel.
In the groin, the inguinal canal passes through all these layers. The testes can descend through it, and in women the fibrous cord from the uterus runs along it. It is also a weak point where inguinal hernias can form.
During forceful exhalation, the abdominal muscles help push air out of the lungs. They also shield the organs beneath them. Working with the back muscles, they support posture and help define the shape of the torso.
When the glottis is closed and both the thorax and the pelvis are held fixed, these muscles drive coughing, urination, defecation, childbirth, vomiting and singing. With the pelvis fixed, they can start the trunk moving forward, and they guard against hyperextension. With the thorax fixed, they can pull the pelvis upward. They can also bend the spine sideways and assist in rotating the trunk.
The deepest of them, the transverse abdominis, cannot be touched from outside the body, yet it greatly affects posture. The internal obliques are also deep, and both muscles take part in rotating and sideways bending of the spine. They bend and support the spine from the front. The more superficial external obliques share those movements and steady the spine when a person stands upright. The rectus abdominis flexes the back forward, and bending the spine forward is the main work of the group when it contracts.
Exercise matters because weak or overly tight abdominal and back muscles can suffer painful spasms and injuries. Well-conditioned muscles improve posture and balance, make back pain less frequent and less severe, and protect against injury. They can help people avoid some back surgeries, and they aid healing after spine surgery. Pilates, yoga, tai chi and jogging all work these muscles.
To find these muscles on a living body, clinicians use a set of surface landmarks, a map they rely on every day.
Roughly halfway down a slight furrow in the mid-line sits the navel, also called the umbilicus. The furrow runs from the xiphoid process down to the pubic symphysis and marks the linea alba beneath the skin. In muscular people the rectus abdominis stands out on each side of it. Three or more transverse depressions interrupt its outline. Usually one lies near the xiphoid process, one at the navel and one between them.
The upper side limit of the abdomen is the subcostal margin, formed where the cartilages of ribs 8, 9 and 10 meet. The lower side limit follows the anterior crest of the ilium and Poupart's ligament, and visible grooves mark it. Just above the pubic spines are the external abdominal rings. They are openings in the muscular wall through which the spermatic cord emerges in the male.
To locate what lies beneath, anatomists draw three horizontal and two vertical lines. The highest horizontal line is the transpyloric line of C. Addison. It lies halfway between the suprasternal notch and the top of the pubic symphysis and corresponds to the first lumbar vertebra behind. It often cuts the pyloric opening of the stomach an inch to the right of the mid-line. The hilum of each kidney sits a little below it.
The second line, the subcostal line, starts from the lowest point of the tenth rib. It matches the upper part of the third lumbar vertebra and lies an inch or so above the umbilicus. It roughly indicates the transverse colon and the lower ends of the kidneys. The third, the intertubercular line, crosses between two tubercles felt on the iliac crest. It corresponds to the fifth lumbar vertebra and passes at or just above the ileo-caecal valve, where small intestine meets large.
The two vertical lines are the mid-Poupart lines, which rise from the midpoint between the anterior superior spine and the pubic symphysis up to the costal margin. The right one is the more valuable. The ileo-caecal valve sits where it crosses the intertubercular line, and the orifice of the appendix lies an inch lower, at McBurney's point. Higher up, it meets the transpyloric line near the ninth rib, where the gallbladder is found. The left line follows the inner edge of the descending colon. The right subcostal margin matches the lower limit of the liver.
A simpler scheme splits the abdomen into four quadrants, left upper, left lower, right upper and right lower. Doctors often use them to describe where abdominal pain is felt. A finer scheme uses nine regions, including the epigastric, the umbilical and the hypogastric. Their names come from Greek roots. Hypo means below, epi means above, chondron means cartilage, and gaster means stomach. Left and right are deliberately reversed, because the labels follow the patient's own sides.
One region has a famously imprecise name, the right iliac fossa. It is a common site of pain and tenderness in patients with appendicitis. Most of the structures that cause pain there do not actually lie in the hollow of the iliac bone, yet the term remains in everyday use.
Endoscopy, colonoscopy, sigmoidoscopy, enteroscopy and oesophagogastroduodenoscopy are among the procedures used to examine the gastrointestinal tract. Virtual colonoscopy joins them, and a number of medical imaging techniques are available as well.
Many gastrointestinal diseases strike the abdominal organs. They include diseases of the stomach, liver, pancreas, gallbladder and bile ducts. Intestinal conditions include enteritis, coeliac disease, diverticulitis and irritable bowel syndrome.
Fat can build up in a particular way as well. Abdominal obesity is an excess of visceral fat packed between the organs. It is associated with a higher risk of heart disease, asthma and type 2 diabetes.
Injury is another danger. Abdominal trauma can damage the organs and carries a risk of severe blood loss and infection. A blow to the lower chest can injure the spleen and liver.
The shape of the abdomen can also give a clue. A scaphoid abdomen is one sucked inward, and in general it points to malnutrition. In a newborn, though, it may signal a diaphragmatic hernia, a fault in the very diaphragm that forms the cavity's upper edge.
Cattle and sheep belong to the ruminants, and their stomach is divided into four chambers called the rumen, reticulum, omasum and abomasum. Among mammals, that is one example of highly specialized abdominal organs.
In arthropods the abdomen is assembled from upper plates called tergites and lower plates called sternites. A tough but stretchable membrane holds the whole structure together.
In insects the abdomen houses the digestive tract and the reproductive organs. Most insect orders have eleven segments, though the eleventh is missing in the adults of most higher orders. The count varies by species. A common honey bee shows only seven segments, and springtails, the Collembola, have six.
Among the Apocrita, the group that includes bees, ants and wasps, the first abdominal segment is fused to the thorax and is called the propodeum. In ants the second segment forms a narrow waist, the petiole. Some ants add a postpetiole segment, and the remaining segments swell into the bulbous gaster. The petiole and gaster together, segments 2 and onward, are the metasoma.
In arachnids such as spiders and scorpions, the abdomen is used interchangeably with the term opisthosoma, meaning hind body. It is the section behind the one carrying the legs and head, which is called the prosoma or cephalothorax.
Adult insects have no legs on the abdomen. The Protura, however, keep rudimentary leg-like appendages on their first three abdominal segments. The Archaeognatha have small articulated styli, which are sometimes treated as rudimentary appendages. Many larvae, including those of Lepidoptera and the sawflies, have fleshy prolegs along the abdomen. Each time a caterpillar grips the edge of a leaf and walks on, it relies on appendages its adult form will no longer have.
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Common questions
Where does the human abdomen begin and end?
It stretches from the thoracic diaphragm at the top to the pelvic brim at the bottom. The brim runs from the disc between the L5 and S1 vertebrae to the pubic symphysis.
What muscles form the abdominal wall?
Three layers form the wall. From the outside in they are the external oblique, the internal oblique and the transverse abdominal. The rectus abdominis runs vertically within a sheath formed by their fibers.
Is the rectus abdominis the outermost abdominal muscle?
No. A sheath extending from the external obliques covers it, even though it is the muscle that very fit people develop into a six-pack.
How many segments does an insect abdomen have?
Most insect orders have eleven segments, though the eleventh is absent in the adults of most higher orders. A honey bee shows only seven, and springtails have six.
What is the opisthosoma?
It is the arachnid term used interchangeably with abdomen. The word means hind body, and it names the section behind the one bearing the legs and head.
All sources
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