Abdomen

The abdomen (less formally called the belly,[1] stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the frontal part of the abdominal segment of the trunk, the dorsal part of this segment being the back of the abdomen. The region occupied by the abdomen is termed the abdominal cavity. In arthropods it is the posterior tagma of the body; it follows the thorax or cephalothorax.[2][3] 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.

Abdomen
Anatomy Abdomen Tiesworks
The human abdomen and organs which can be found beneath the surface
Abdomen-periumbilical region
Details
InsertionRib cage
Vertebral column
ActionsMovement and support for the torso
Assistance with breathing
Protection for the inner organs
Postural support
Identifiers
LatinAbdomen
MeSHD000005
TAA01.1.00.016
FMA9577
Anatomical terminology

Structure

Contents

Gray1120
The relations of the viscera and large vessels of the abdomen, seen from behind.

The abdomen contains most of the tubelike organs of the digestive tract, as well as several solid organs. Hollow abdominal organs include the stomach, the small intestine, and the colon with its attached appendix. Organs such as the liver, its attached gallbladder, and the pancreas function in close association with the digestive tract and communicate with it via ducts. The spleen, kidneys, and adrenal glands also lie within the abdomen, along with many blood vessels including the aorta and inferior vena cava. Anatomists may consider the urinary bladder, uterus, fallopian tubes, and ovaries 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. Anatomists call the latter type of organs retroperitoneal.

Abdominal organs can be highly specialized in some animals. For example, the stomach of ruminants (a suborder of mammals) is divided into four chambers – rumen, reticulum, omasum and abomasum.[4] In vertebrates, the abdomen is a large cavity enclosed by the abdominal muscles, ventrally and laterally, and by the vertebral column dorsally. Lower ribs can also enclose ventral and lateral walls. The abdominal cavity is upper part of 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.[5] The abdomen in vertebrates contains a number of organs belonging, for instance, to the digestive tract and urinary system.

Sobo 1906 393

View of the various organs and blood-vessels in proximity with liver.

Sobo 1906 405

Liver lifted to show gall bladder and stomach in situ.

Muscles

Grays Anatomy image392
Example of a physically fit human male abdomen
(Left) Henry Gray (1825–1861). Anatomy of the Human Body.
(Right) A male abdomen

There are three layers of the abdominal wall. They are, from the outside to the inside: external oblique, internal oblique, and transverse abdominal.[6] 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 are downward and forward, the internal oblique upward and forward, and the transverse abdominal horizontally forward.[6]

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, 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.[6]

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

Abdominal Organs Anatomy
Abdominal organs anatomy.

Functionally, the human abdomen is where most of the alimentary 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 in the breathing process as accessory muscles of respiration. 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.[6] 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.[6]

Posture

The transverse abdominis muscle is the deepest muscle, therefore, it cannot be touched from the outside. It can greatly affect the body 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.[7]

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 spinesl 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 healing from 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.[8] Injury to the lower chest can cause injuries to the spleen and liver.[9]

A scaphoid abdomen is when the abdomen is sucked inwards.[10] In a newborn, it may represent a diaphragmatic hernia.[11] In general, it is indicative of malnutrition.[12]

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

Surface projections of the organs of the trunk
Surface projections of the organs of the trunk, from which organ locations are derived mainly from vertebra levels, ribs and the ilium.

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.

Gray1225
Front of abdomen, showing markings for duodenum, pancreas, and kidneys.
  • 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 two and a half inches (64 mm) 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.

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.

Regions

Gray1220
Surface lines of the front of the thorax and abdomen.

These three horizontal and two vertical lines divide the abdomen into nine "regions." (Note that "hypo" means "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.)

right hypochondriac/hypochondrium epigastric/epigastrium left hypochondriac/hypochondrium
right lumbar/flank/latus/lateral umbilical left lumbar/flank/latus/lateral
right inguinal/iliac hypogastric/suprapubic left inguinal/iliac

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.

Blausen 0005 AbdominopelvicQuadrants
Quadrants of the abdomen.

Another way of dividing the abdomen is by using 4 quadrants:

right upper quadrant (RUQ)

left upper quadrant (LUQ)

right lower quadrant (RLQ)

left lower quadrant (LLQ)

Abdominal Quadrant Regions
Side-by-side comparison of 4- and 9-region schemes.

Other animals

Abdomen (PSF)
The analogous gross morphologies of a human and an ant
Scheme ant worker anatomy-en
In the worker ant, the abdomen consists of the propodeum fused to the thorax and the metasoma, itself divided into the narrow petiole and bulbous gaster.

The invertebrate abdomen is built up of a series of upper plates known as tergites and lower plates known as sternites, 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.[13] 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

References

  1. ^ CHRISTINA STIEHL(12/21/2017). What Actually Causes a Beer Belly, and How to Get Rid of It. Thrillist. Access date [4 Feb 2019].
  2. ^ Abdomen. (n.d.). Dictionary.com Unabridged (v 1.1). Accessed: 22 October 2007
  3. ^ Abdomen. Dictionary.com. The American Heritage Dictionary of the English Language, 4th Edition. Accessed: 22 October 2007
  4. ^ "Ruminant." The Veterinary Dictionary. Elsevier, 2007. Accessed: 22 October 2007
  5. ^ Peritoneum. The Veterinary Dictionary. Elsevier, 2007. Accessed: 22 October 2007
  6. ^ a b c d e "Abdominal cavity". Encyclopædia Britannica. I: A-Ak – Bayes (15th ed.). Chicago, IL: Encyclopædia Britannica, Inc. 2010. pp. 19–20. ISBN 978-1-59339-837-8.
  7. ^ "The Abdominal Muscle Group". Retrieved 2010-07-13.
  8. ^ Jansen JO, Yule SR, Loudon MA (April 2008). "Investigation of blunt abdominal trauma". BMJ. 336 (7650): 938–42. doi:10.1136/bmj.39534.686192.80. PMC 2335258. PMID 18436949.
  9. ^ Wyatt, Jonathon; Illingworth, RN; Graham, CA; Clancy, MJ; Robertson, CE (2006). Oxford Handbook of Emergency Medicine. Oxford University Press. p. 346. ISBN 978-0-19-920607-0.
  10. ^ al.], consultants Daniel Albert ... [et (2012). Dorland's illustrated medical dictionary (32nd ed.). Philadelphia, PA: Saunders/Elsevier. p. 2. ISBN 978-1-4160-6257-8.
  11. ^ Durward, Heather; Baston, Helen (2001). Examination of the newborn: a practical guide. New York: Routledge. p. 134. ISBN 978-0-415-19184-5.
  12. ^ Ferguson, Charles (1990). "Chapter 93: Inspection, Auscultation, Palpation, and Percussion of the Abdomen". In Walker, HK; Hall, WD; JW, Hurst (eds.). Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed.). Boston: Butterworths. Retrieved 2013-11-27.
  13. ^ "Glossary of Descriptive Terminology". Desertants.org. Archived from the original on 17 May 2013. Retrieved 2013-07-08.

External links

Abdominal distension

Abdominal distension occurs when substances, such as air (gas) or fluid, accumulate in the abdomen causing its expansion. It is typically a symptom of an underlying disease or dysfunction in the body, rather than an illness in its own right. People suffering from this condition often describe it as "feeling bloated". Sufferers often experience a sensation of fullness, abdominal pressure and possibly nausea, pain or cramping. In the most extreme cases, upward pressure on the diaphragm and lungs can also cause shortness of breath. Through a variety of causes (see below), bloating is most commonly due to buildup of gas in the stomach, small intestine or colon. The pressure sensation is often relieved, or at least lessened, by belching or flatulence. Medications that settle gas in the stomach and intestines are also commonly used to treat the discomfort and lessen the abdominal distension.

Abdominal external oblique muscle

The external oblique muscle (of the abdomen) (also external abdominal oblique muscle) is the largest and the outermost of the three flat muscles of the lateral anterior abdomen.

Abdominal pain

Abdominal pain, also known as a stomach ache, is a symptom associated with both non-serious and serious medical issues.

Common causes of pain in the abdomen include gastroenteritis and irritable bowel syndrome. About 10% of people have a more serious underlying condition such as appendicitis, leaking or ruptured abdominal aortic aneurysm, diverticulitis, or ectopic pregnancy. In a third of cases the exact cause is unclear.Given that a variety of diseases can cause some form of abdominal pain, a systematic approach to examination of a person and the formulation of a differential diagnosis remains important.

Abdominal wall

In anatomy, the abdominal wall represents the boundaries of the abdominal cavity. The abdominal wall is split into the posterior (back), lateral (sides) and anterior (front) walls.

There is a common set of layers covering and forming all the walls: the deepest being the visceral peritoneum, which covers many of the abdominal organs (most of the large and small intestines, for example), and the parietal peritoneum- which covers the visceral peritoneum below it, the extraperitoneal fat, the transversalis fascia, the internal and external oblique and transversus abdominis aponeurosis, and a layer of fascia, which has different names according to what it covers (e.g., transversalis, psoas fascia).

In medical vernacular, the term 'abdominal wall' most commonly refers to the layers composing the anterior abdominal wall which, in addition to the layers mentioned above, includes the three layers of muscle: the transversus abdominis (transverse abdominal muscle), the internal (obliquus internus) and the external oblique (obliquus externus).

Abdominal x-ray

An abdominal x-ray is an x-ray of the abdomen. It is sometimes abbreviated to AXR, or KUB (for kidneys, ureters, and urinary bladder).

Aorta

The aorta ( ay-OR-tə) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes oxygenated blood to all parts of the body through the systemic circulation.

Ascites

Ascites is the abnormal buildup of fluid in the abdomen. Technically, it is more than 25 mL of fluid in the peritoneal cavity. Symptoms may include increased abdominal size, increased weight, abdominal discomfort, and shortness of breath. Complications can include spontaneous bacterial peritonitis.In the developed world, the most common cause is liver cirrhosis. Other causes include cancer, heart failure, tuberculosis, pancreatitis, and blockage of the hepatic vein. In cirrhosis, the underlying mechanism involves high blood pressure in the portal system and dysfunction of blood vessels. Diagnosis is typically based on an examination together with ultrasound or a CT scan. Testing the fluid can help in determining the underlying cause.Treatment often involves a low salt diet, medication such as diuretics, and draining the fluid. A transjugular intrahepatic portosystemic shunt (TIPS) may be placed but is associated with complications. Effects to treat the underlying cause, such as by a liver transplant may be considered. Of those with cirrhosis, more than half develop ascites in the ten years following diagnosis. Of those in this group who develop ascites, half will die within three years. The term is from the Greek askítes meaning "baglike".

Crab

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) (Greek: βραχύς, translit. brachys = short, οὐρά / οura = tail), usually entirely hidden under the thorax. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice – are not true crabs.

Heartburn

Heartburn, also known as pyrosis, cardialgia or acid indigestion, is a burning sensation in the central chest or upper central abdomen. The discomfort often rises in the chest and may radiate to the neck, throat, or angle of the jaw.

Heartburn is usually due to regurgitation of gastric acid (gastric reflux) into the esophagus and is the major symptom of gastroesophageal reflux disease (GERD). In about 0.6% of cases it is a symptom of ischemic heart disease.

Ileus

Ileus is a disruption of the normal propulsive ability of the intestine due to the malfunction of peristalsis.

Ileus originally referred to any lack of digestive propulsion, including bowel obstruction,

but current medical usage restricts its meaning only to those disruptions caused by the failure of the system's peristalsis and excludes failures due to mechanical obstruction,

with the exception of certain older terms such as "gallstone ileus" and "meconium ileus" which are still accepted as correct (though they are now technically misnomers) due to their persistence in usage.

The word 'ileus' is from Greek εἰλεός eileós, "intestinal obstruction".

Laparotomy

A laparotomy is a surgical procedure involving a large incision through the abdominal wall to gain access into the abdominal cavity. It is also known as a celiotomy. The first successful laparotomy was performed without anesthesia by Ephraim McDowell in 1809 in Danville, Kentucky.

Nausea

Nausea is an unpleasant, diffuse sensation of unease and discomfort, often perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged, and has been described as placing discomfort on the chest, upper abdomen, or back of the throat.Like pain, the purpose of nausea is to discourage the person or animal from repeating whatever caused the unpleasantness. The memory of pain elicits safer or evasive actions; the memory of nausea elicits revulsion towards whatever was eaten before vomiting it up — even if it was not the cause of the nausea.Nausea is a non-specific symptom, which means that it has many possible causes. Some common causes of nausea are gastroenteritis and other gastrointestinal disorders, food poisoning, motion sickness, dizziness, migraine, fainting and low blood sugar. Nausea is a side effect of many medications including chemotherapy, or morning sickness in early pregnancy. Nausea may also be caused by anxiety, disgust and depression.Medications taken to prevent and treat nausea are called antiemetics. The most commonly prescribed antiemetics in the US are promethazine, metoclopramide and the newer, extremely effective ondansetron. The word nausea is from Latin nausea, from Greek ναυσία – nausia, "ναυτία" – nautia, motion sickness, "feeling sick or queasy".

Navel

The navel (clinically known as the umbilicus, colloquially known as the belly button, or tummy button) is a protruding, flat, or hollowed area on the abdomen at the attachment site of the umbilical cord. All placental mammals including humans have a navel.

Opisthothelae

The Opisthothelae are spiders within the order Araneae, consisting of the Mygalomorphae and the Araneomorphae, but excluding the Mesothelae. The Opisthothelae are sometimes presented as an unranked clade and sometimes as a suborder of the Araneae. In the latter case, the Mygalomorphae and Araneomorphae are treated as infraorders.

The fairly recent creation of this taxon has been justified by the requirement to distinguish these spiders from the Mesothelae, which display many more primitive characteristics. Those that distinguish between the Mesothelae and Opisthothelae are:

The tergite plates on the abdomen of Mesothelae but absent in Opisthothelae

The almost total absence of ganglia in the abdomen of Opisthothelae

The almost median position of the spinnerets in the Mesothelae compared with the hindmost position of those of the OpistothelaeAmong the Opisthothelae, the fangs of the Mygalomorphae point straight down in front of the mouth aperture and only allow the spider to grasp its prey from above and below, whereas in the Araneomorphae, they face one another like pincers, allowing a firmer grip. Distinguishing araneomorphs and mygalomorphs on first inspection is difficult unless the specimens are large enough to permit immediate examination of the fangs.

Peritoneum

The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesothelium supported by a thin layer of connective tissue. This peritoneal lining of the cavity supports many of the abdominal organs and serves as a conduit for their blood vessels, lymphatic vessels, and nerves.

The abdominal cavity (the space bounded by the vertebrae, abdominal muscles, diaphragm, and pelvic floor) is different from the intraperitoneal space (located within the abdominal cavity but wrapped in peritoneum). The structures within the intraperitoneal space are called "intraperitoneal" (e.g., the stomach and intestines), the structures in the abdominal cavity that are located behind the intraperitoneal space are called "retroperitoneal" (e.g., the kidneys), and those structures below the intraperitoneal space are called "subperitoneal" or "infraperitoneal" (e.g., the bladder).

Peritonitis

Peritonitis is inflammation of the peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One part or the entire abdomen may be tender. Complications may include shock and acute respiratory distress syndrome.Causes include perforation of the intestinal tract, pancreatitis, pelvic inflammatory disease, stomach ulcer, cirrhosis, or a ruptured appendix. Risk factors include ascites and peritoneal dialysis. Diagnosis is generally based on examination, blood tests, and medical imaging.Treatment often includes antibiotics, intravenous fluids, pain medication, and surgery. Other measures may include a nasogastric tube or blood transfusion. Without treatment death may occur within a few days. Approximately 7.5% of people have appendicitis at some point in time. About 20% of people with cirrhosis who are hospitalized have peritonitis.

Quadrant (abdomen)

The human abdomen is divided into regions by anatomists and physicians for purposes of study, diagnosis, and therapy. In the four-region scheme, four quadrants allow localisation of pain and tenderness, scars, lumps, and other items of interest, narrowing in on which organs and tissues may be involved. The quadrants are referred to as the left lower quadrant, left upper quadrant, right upper quadrant and right lower quadrant, as follows below. These terms are not used in comparative anatomy, since most other animals do not stand erect.

The left lower quadrant (LLQ) of the human abdomen is the area left of the midline and below the umbilicus. The LLQ includes the left iliac fossa and half of the left flank region. The equivalent term for animals is left posterior quadrant.

The left upper quadrant (LUQ) extends from the median plane to the left of the patient, and from the umbilical plane to the left ribcage. The equivalent term for animals is left anterior quadrant.

The right upper quadrant (RUQ) extends from the median plane to the right of the patient, and from the umbilical plane to the right ribcage. The equivalent term for animals is right anterior quadrant.

The right lower quadrant (RLQ) extends from the median plane to the right of the patient, and from the umbilical plane to the right inguinal ligament. The equivalent term for animals is right posterior quadrant.

Spider

Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs able to inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization. As of November 2015, at least 45,700 spider species, and 113 families have been recorded by taxonomists. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900.Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. Unlike insects, spiders do not have antennae. In all except the most primitive group, the Mesothelae, spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure.

Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of glands. Spider webs vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-web spiders. Spider-like arachnids with silk-producing spigots appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving suborder, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago.

The species Bagheera kiplingi was described as herbivorous in 2008, but all other known species are predators, mostly preying on insects and on other spiders, although a few large species also take birds and lizards. It is estimated that the world's 25 million tons of spiders kill 400–800 million tons of prey per year. Spiders use a wide range of strategies to capture prey: trapping it in sticky webs, lassoing it with sticky bolas, mimicking the prey to avoid detection, or running it down. Most detect prey mainly by sensing vibrations, but the active hunters have acute vision, and hunters of the genus Portia show signs of intelligence in their choice of tactics and ability to develop new ones. Spiders' guts are too narrow to take solids, so they liquefy their food by flooding it with digestive enzymes. They also grind food with the bases of their pedipalps, as arachnids do not have the mandibles that crustaceans and insects have.

To avoid being eaten by the females, which are typically much larger, male spiders identify themselves to potential mates by a variety of complex courtship rituals. Males of most species survive a few matings, limited mainly by their short life spans. Females weave silk egg-cases, each of which may contain hundreds of eggs. Females of many species care for their young, for example by carrying them around or by sharing food with them. A minority of species are social, building communal webs that may house anywhere from a few to 50,000 individuals. Social behavior ranges from precarious toleration, as in the widow spiders, to co-operative hunting and food-sharing. Although most spiders live for at most two years, tarantulas and other mygalomorph spiders can live up to 25 years in captivity.

While the venom of a few species is dangerous to humans, scientists are now researching the use of spider venom in medicine and as non-polluting pesticides. Spider silk provides a combination of lightness, strength and elasticity that is superior to that of synthetic materials, and spider silk genes have been inserted into mammals and plants to see if these can be used as silk factories. As a result of their wide range of behaviors, spiders have become common symbols in art and mythology symbolizing various combinations of patience, cruelty and creative powers. An abnormal fear of spiders is called arachnophobia.

Spleen

The spleen is an organ found in virtually all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes from Ancient Greek σπλήν (splḗn).The spleen plays important roles in regard to red blood cells (erythrocytes) and the immune system. It removes old red blood cells and holds a reserve of blood, which can be valuable in case of hemorrhagic shock, and also recycles iron. As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent red blood cells (erythrocytes). The globin portion of hemoglobin is degraded to its constitutive amino acids, and the heme portion is metabolized to bilirubin, which is removed in the liver.The spleen synthesizes antibodies in its white pulp and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation. A study published in 2009 using mice found that the red pulp of the spleen forms a reservoir that contains over half of the body's monocytes. These monocytes, upon moving to injured tissue (such as the heart after myocardial infarction), turn into dendritic cells and macrophages while promoting tissue healing. The spleen is a center of activity of the mononuclear phagocyte system and is analogous to a large lymph node, as its absence causes a predisposition to certain infections.In humans the spleen is purple in color and is in the left upper quadrant of the abdomen.

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