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Anatomy
of the Gastrointestinal Tract
Gastrointestinal
(GI) Tract
The gastrointestinal (GI) tract (also digestive tract,
or alimentary canal) is the system of organs in the
body that takes in food, digests it for the absorption
of nutrients and energy, and expels waste material.
The major functions of the GI tract are categorized
as four distinct processes:
- Ingestion
is the consumption of food and other substances through
the mouth, as they pass by chewing and swallowing
into the GI tract.
- Digestion
is the process of metabolism by which ingested substances
are mechanically and chemically converted for use
by the body. Digestion is further categorized into
three distinct phases: the cephalic phase in which
taste and smell stimulate the nervous system to prepare
the body for eating and digestion; the gastric phase
in which passage of food into the stomach stimulates
the release of gastric juices and pH balancing mechanisms
throughout the system; the intestinal phase in which
excitatory and inhibitory reflexes control the passage
of partially digested food into and through the intestines.
- Absorption
is the movement of metabolized nutrients and water
from the digestive system into the circulatory and
lymphatic capillaries by osmosis, active transport,
and diffusion through the cells in the walls and surrounding
layers of the intestines and their supporting circulatory
systems.
- Excretion
or egestion is the elimination of undigested, mostly
solid material from the GI tract by defecation. Fluid
products of metabolism throughout the body are also
excreted by organ systems not directly part of the
GI tract and digestive system, such as the kidneys,
skin, and lungs.
In addition to processing nutrients as the principal
pathway of the digestive system, the GI tract is also
a prominent part of the immune system, providing various
levels of defense against pathogenic microorganisms
and potentially toxic substances throughout the path
of digestion. Dysfunction anywhere in the GI tract,
whether by disease, trauma, or anatomical anomaly, can
result in symptoms or conditions affecting the well-being
of the entire individual. Many diseases and disorders
of the GI tract can result in feeding difficulties in
children and infants.
Digestive System Diagram
The GI tract is conventionally divided into upper and
lower parts, with associated accessory organs.
Upper GI Tract
The upper GI tract consists of the mouth, pharynx, esophagus,
and stomach. This is where ingestion and the first phase
of digestion occurs.
- The
mouth includes the tongue, teeth,
and buccal mucosa or mucous membranes containing the
ends of the salivary glands, continuous with the soft
palate, floor of the mouth and underside of the tongue.
Chewing (mastication) is the mechanical process by
which food, constantly repositioned by muscular action
of the tongue and cheeks, is crushed and ground by
the teeth through the muscular action of the lower
jaw (mandible) against the fixed resistance of the
upper jaw (maxilla). Saliva excreted in the oral cavity
by three pairs of exocrine glands (parotid, submandibular,
and sublingual) is mixed with chewed food to form
a bolus, or ball-shaped mass. There are two types
of saliva: a thin watery secretion that wets the food
and a thick mucous secretion that lubricates and causes
the food particles to stick together to form the bolus.
Digestive enzymes in saliva begin the chemical breakdown
of food, primarily starches at this point, almost
immediately.
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Head and Neck and Isthmus of the Fauces
- The
pharynx is contained in the neck
and throat and functions as part of both the digestive
system and the respiratory system. The human pharynx
is divided into three sections: the nasopharynx behind
the nasal cavity and above the soft palate; the oropharynx
behind the oral cavity and including the base of the
tongue, the tonsils, and the uvula; the hypopharynx
or laryngopharynx includes the junction with the esophagus
and the larynx, where respiratory and digestive pathways
diverge. The swallowing reflex is initiated by touch
receptors in the pharynx as the bolus of chewed food
is pushed to the back of the mouth. Swallowing automatically
closes down the respiratory or breathing pathway as
an anti-choking reflex. Failure or confusion of reflexes
at this point can result in aspiration of solid or
liquid food into the trachea and lungs.
- The
esophagus is the hollow muscular
tube through which food passes from the pharynx to
the stomach. It is also lined with mucous membrane
continuous with the mucosa of the mouth and into which
open the esophageal glands. The esophagus is surrounded
by relatively deep muscles that move the swallowed
bolus of masticated food through peristaltic action,
piercing the thoracic diaphragm to reach the stomach.
- The
stomach is a hollow muscular organ,
located below the diaphragm and above the small intestine,
that receives and holds masticated food to begin the
next phase of digestion. Two smooth muscle valves,
the esophageal sphincter above and the pyloric sphincter
below, keep stomach contents contained. The stomach
is surrounded by stimulant (parasympathetic) and inhibitor
(orthosympathetic) nerve plexuses which regulate both
secretory and muscular activity during digestion.
With a volume of as little as 50 mL when empty, the
adult human stomach may comfortably contain about
a liter of food after a meal, or uncomfortably as
much as 4 liters of liquid.
Diagram of the Stomach
Lower GI Tract
The lower GI tract includes the small intestine and
large intestine, beginning after the stomach and terminating
at the anus. Its function is to complete the digestion
and absorption of nutrients and to prepare waste products
for elimination from the digestive system.
Small
Intestine
The small intestine is where most digestion takes place.
It is structurally divided into three parts: the duodenum,
the jejunum, and the ileum. Among humans over five years
old, the small intestine tends to vary in length from
4-7 meters (13 to 23 feet).
Small Intestine
- The
duodenum consists of four parts,
with the first three forming a “C” shape.
The first or superior part of the duodenum begins
at the pylorus, passing laterally for a short distance
before curving into the superior duodenal flexure.
The second or descending part the duodenum passes
from the superior into the inferior duodenal flexure,
and is where the pancreatic and common bile ducts
enter the GI tract. The third or inferior horizontal
part of the duodenum passes from the inferior flexure,
crossing the aorta (major artery) and inferior vena
cava (major vein), as well as the spinal column. The
fourth or ascending part of the duodenum passes over
the aorta, and curves past the pancreas to the duodenojejunal
flexure. The duodenum is where most of the breakdown
of food in the small intestine occurs. It is here
that Brunner’s glands produce an alkaline secretion
to protect the duodenum from acidic chyme entering
from the stomach and to activate intestinal enzymes
enabling digestion and absorption.
Duodenum-Brunner’s Glands
- The
jejunum begins at the ligament of
Treitz in the duodenojejunal flexure and continues
to the ileum. The inner surface or mucous membrane
of the jejunum is covered by villi (small finger-like
structures) much longer than found in the duodenum
or ileum, contained in many large circular folds (plicae
circulares) which provide extensive surface area for
absorption of nutrients. The villi can increase intestinal
absorptive surface area by a factor of 30; the microvilli
extensions of the villi increase surface area by an
additional factor of 600. Villus capillaries collect
amino acids and simple sugars. Villus lacteals or
lymphatic capillaries absorb dietary fats.
Microvilli
- The
ileum is the final and longest section
of the small intestine. Both the jejunum and the ileum
are suspended by mesentery, a double layer of peritoneum
that allows these parts of the intestine to move more
freely within the abdomen. Like the jejunum, the wall
of the ileum has many folds and villi to increase
both adsorption of enzymes and absorption of nutrients.
It also has an increasing number of goblet cells.
The ileum is responsible for the final stages of protein
and carbohydrate digestion, as contents are pushed
along by peristaltic waves of smooth muscle contractions.
There is no absolute demarcation between the jejunum
and the ileum, but the ileum tends to have more fat
inside the mesentery and has a relatively decreasing
diameter. Unlike the rest of the small intestine,
the ileum has abundant Peyer’s patches, lymphoid
follicles similar to lymph nodes, which function as
an important component of the immune system response
to pathogenic organisms in the GI tract.
Goblet Cells
Large Intestine
Also commonly referred to by the name of its longest
component, the colon, the large intestine is the last
part of the digestive system. Its principal function
is to absorb remaining water from the waste products
of digestion as it compacts the accumulated waste for
periodic elimination by defecation. While food is not
broken down further at this stage, the fluid absorption
function of the large intestine does act to gather in
vitamins created by beneficial bacteria or flora inhabiting
the colon. Instead of the predominance of evaginations
of villi found in the small intestine, the large intestine
has increased invaginations of glands and an abundance
of goblet cells. The large intestine is structurally
divided into three parts: cecum, colon, and rectum.
Large Intestine
- The
cecum is a pouch at the beginning
of the large intestine, separated from the ileum of
the small intestine by the ileocecal valve and joining
the colon at the cecocolic junction in the lower right
quadrant of the abdomen. The cecum is host to a large
number of bacteria which aid in the final enzymatic
processing of material not completely digested in
the small intestine. The vermiform appendix is a worm-like
cul-de-sac attachment of the cecum, until recently
considered entirely vestigial in humans, but now thought
to have a role as a haven for the beneficial gut flora,
as well as a site of infection-fighting lymphoid cells.
Gray’s Cecum
- The
colon consists of four parts named
for their relative orientation in the abdomen: (1)
the ascending colon, (2) the transverse colon, (3)
the descending colon, and the (4) sigmoid colon. By
the time chyme has reached the colon, almost all nutrients
and most of the water have already been absorbed by
the body. It is here that the chyme is mixed with
mucus and bacteria to become feces. The waste products
of bacterial metabolism include some nutrients used
by the cells lining the colon for their own nourishment.
The colon ends at the junction of the sigmoid colon
and (5) the rectum.
 
Colon
- The
rectum is the last part of the large intestine, beginning
at and continuous with the colon, and terminating
at anus. The rectum provides temporary storage for
feces. Stretch receptors of the nervous system located
in the rectal walls stimulate the desire to defecate.
As peristaltic waves propel the feces into the anal
canal, external and internal sphincters allow the
final exit of waste material from the GI tract.
Anorectum
Accessory Organs
Accessory to the alimentary canal of the GI tract are
various secretory, storage, and waste filtering organs
and related hormonal glands. Principal among these are
the liver, gallbladder, and pancreas.
-
The liver secretes bile, produced
by its hepatocytes, into the duodenum of the small
intestine via the biliary system. Bile acts as a kind
of detergent, emulsifying fats to promote enzyme action
in the intestines. Epithelial cells in the liver add
a watery solution rich in bicarbonates that act to
dilute and neutralize acids at this stage of digestion.
Cholesterol is also released with the bile and is
important for the metabolism of fat soluble vitamins
as well as maintenance of normal cell membranes throughout
the body. Consistent with its major role in metabolism,
the liver has a number of functions not strictly related
to digestion, such as decomposition of red blood cells,
plasma protein synthesis, and detoxification. The
liver is the largest gland in the human body and performs
or regulates a wide variety of high-volume reactions
involving very specialized tissues.
Liver
Biliary System
- The
gallbladder is connected to the liver and the duodenum
by the biliary tract. The gallbladder stores the bile
(or gall) secreted by the liver until its release
is triggered by the digestive process. The interior
of the gallbladder has a simple columnar epithelial
lining characterized by recesses or pouches, which
provide the volume for storage. The cystic duct connects
the gallbladder to common hepatic duct to form the
common bile duct.
Gallbladder and Simple Columnar Epithelium
- The
pancreas is another relatively large
gland that functions as part of both the digestive
and endocrine system. Its exocrine function is to
produce and secrete pancreatic juice rich in digestive
enzymes. Its endocrine functions include the production
of important hormones, such as insulin (which helps
regulate metabolism at the global and cellular level),
glucagon (which acts opposite insulin), and somatostatin
(which acts to suppress the release of various other
GI hormones and lower the rate of gastric emptying
as digestion approaches completion). The pancreatic
duct joins the common bile duct, together entering
the major duodenal papilla through the hepatopancreatic
ampulla. Because the human pancreas is not completely
matured until age two, otherwise normal infants will
have digestive problems with entire categories of
foods until all the appropriate enzymes can be properly
secreted by the fully developed pancreas.
Pancreas
GI Histology
In addition to the sequential phases of digestion, and
the corresponding structure and function of each specific
organ in the digestive system, the GI tract is characterized
as having four concentric layers of tissue with functional
differences in each specialized region.
- The
mucosa (moist linings of mucous membranes)
is the first main layer, consisting of the primary
epithelium (tissues lining the cavities and surfaces
of structures throughout the body), the lamina propria
(containing capillaries, lymph vessels, and glands
with ducts opening on to the epithelium), and the
muscularis mucosae (a thin layer of smooth muscle).
- The
submucosa consists of fibrous connective
tissue, separating the mucosa from the surrounding
muscles, and includes fine bundles of nerve plexuses.
- The
muscularis externa (external muscle
layer) generally has two distinct layers of smooth
muscle, the inner (circular) and outer (longitudinal).
In the stomach, there is a third layer (inner oblique)
responsible for the churning or mechanical breakdown
of food. In the esophagus, part of the external muscle
layer is skeletal muscle rather than smooth muscle.
The pyloric and anal sphincters are also formed by
the inner layer of the muscularis externa.
- The
serosa (serous membrane) consists
of layers of connective tissue continuous with the
peritoneum, which forms the lining of the abdominal
cavity and serves as conduit for blood vessels, lymph
vessels, and nerves serving the contained organs.
Pediatric Diseases and Disorders
of GI Anatomy
Diseases or disorders of the mouth and pharynx affecting
feeding can include: strep throat, tonsillitis, mumps,
chronic sore throat, cleft palate or the more general
velocardiofacial syndrome (VCFS), temporomandibular
disorders (TMD), and the entire spectrum of dental disease
and disorders.
Diseases or disorders of the esophagus affecting feeding
can include: peptic strictures, atresia with or without
tracheoesophageal fistula, achlasia, ingestion or aspiration
of foreign or caustic material, and gastrointestinal
reflux disease (GERD)*.
Diseases or disorders of the stomach affecting feeding
can include: dyspepsia, stomach ache, chronic vomiting
syndrome (CVS)**, peptic gastric ulcer, achlrohydria,
hypochlorhydria, hyperchlorhydria, linitis plastica
or Brinton’s disease (a form of stomach cancer),
Zollinger-Ellison syndrome (extremely rare in children),
gastroparesis**, gastroenteritis**, borborygmus, and
GERD*.
Diseases or disorders of the small intestine (comprised
by the duodenum, jejunum, and ileum) affecting feeding
can include: peptic duodenal ulcer, atresia, alimentary
hypoglycemia, celiac disease, rapid gastric emptying
(dumping syndrome, early and late), connective tissue
conditions such as Ehlers-Danlos syndrome, various infectious
diseases (such as giardiasis, scariasis, tropical sprue,
or tapeworm), gastroenteritis**, and other imbalances
of intestinal flora.
Diseases or disorders of the large intestine (comprised
by the cecum, colon, and rectum) affecting feeding can
include: appendicitis, atresia, colitis, colon cancer,
Hirschsprung’s disease**, inflammatory bowel disease
(IBD)**, irritable bowel syndrome (IBS)**, malrotation,
and various other syndromes and conditions manifesting
as either constipation or diarrhea.
Diseases or disorders of the liver affecting feeding
can include: hepatitis, hemochromatosis, cancer, primary
sclerosing cholangitis, Gilbert’s syndrome, and
Wilson’s disease. There are many primarily pediatric
liver diseases, including but by no means limited to:
biliary atresia, antitrypsin deficiency, alagille syndrome,
neonatal jaundice, and progressive intrahepatic cholestasis.
A number of liver function tests are available to evaluate
the proper functioning of the liver by isolating the
presence of certain enzymes in the blood.
Diseases or disorders of the gallbladder affecting feeding
can include: cholestasis, biliary colic, gallstones,
cholecystitis, choledocholithiasis, cancer, and polyps
(including primary sclerosing cholangitis).
Diseases or disorders of the pancreas affecting feeding
can include: diabetes mellitus, benign tumors, cystic
fibrosis, exocrine pancreatic insufficiency, hemosuccus
pancreaticus, acute and chronic pancreatitis, and Zollinger-Ellison
syndrome (extremely rare in children). Since the pancreas
is not fully developed until age two, temporary deficiencies
of digestive enzymes must be considered normal in infants.
Diagnostic Tools
Several tools are available to the physician or specialist
for identifying, evaluating and diagnosing disorders,
malformations, and obstructions throughout the anatomy
of the GI tract in infants, children, and adults. Most
common among these are: biopsy, barium swallow, pH probe,
x-ray, CT scan, and upper or lower endoscopy.
For further details related to GI Anatomy, see
also *REFLUX and **MOTILITY on this website.
A
Closer Look
Appendicitis/Appendectomy
Appendicitis is an acute condition characterized by
inflammation of the vermiform appendix and is recognized
as one of the most common causes of abdominal pain in
humans worldwide. In addition to the immediate symptoms
and effects of acute appendicitis, an appendectomy (surgical
removal of the appendix) can itself have long-term repercussions
on the digestive process and feeding problems
Symptoms of appendicitis can include:
- typical:
initially centralized pain localizing the lower right
side of the abdomen (right iliac fossa), accompanied
by loss of appetite and fever, with or without nausea
and vomiting.
- atypical:
pain beginning and staying in the right iliac fossa,
diarrhea, involvement of the bladder resulting in
frequent micturition or urination.
Causes
of appendicitis can include:
- fecalith,
fecaloma (i.e., tumor made of feces), and coprolith
(i.e., stones made of feces) are different names for
chronic obstructions of the colon by hardening of
the feces. Fecal impaction itself has a variety of
possible causes and other effects which must be considered
in a diagnosis of appendicitis, but a fecalith is
usually the cause of acute appendicitis.
- Other
primary obstruction of the appendix lumen by foreign
bodies, trauma, intestinal worms, or lymphadenitis.
- Low
dietary fiber intake and the resulting increased transit
time of fecal matter has been implicated in the pathogenesis
of appendicitis.
Acute Appendix
Peritonitis is an inflammation of the serous membrane
lining the abdominal cavity and some of the organs contained
within. It may be acute or generalized, but generally
depends on an infection spread by the rupture of an
internal organ, most commonly an appendix. The real
danger of this life-threatening condition is why acute
appendicitis warrants immediate evaluation and treatment.
In children particularly, differential diagnosis must
also consider gastroenteritis, mesenteric adenitis,
Meckel’s diverticulitis, intussusceptions, Henoch-Schonlein
purpura, and lobar pneumonia.
Treatment begins by cessation of all eating or drinking,
even water, for a period before surgery. An intravenous
drip may be used to keep the patient hydrated. Antibiotics
may also be administered intravenously to inhibit the
spread of infection and other postoperative complications.
Surgical removal of the appendix can be accomplished
laparoscopically in simple cases, but may be converted
to open laparotomy if necessary due to rupture, abscess,
or adhesions. It may last from fifteen minutes in typical
appendicitis in thin or small patients to several hours
in complicated cases. Hospital stays range from overnight
to a matter of days, sometimes weeks in rare cases.
The pain is not always constant, sometimes stopping
for a day and then returning.
Most appendicitis patients recover easily, but complications
can ensue if treatment is delayed or peritonitis has
occurred, usually due to a rupture of the infected organ
before treatment. For young children up to ten years
old, complete recovery takes about three weeks, with
minor dietary restrictions. Constipation can be a problem
for a few days following surgery and bowel movements
should be monitored.
Long-term effects of the loss of an appendix are not
considered critical, and there is no reasonable chance
to save the infected organ. A healthy appendix is now
thought to have a key role in recovery of intestinal
flora from flushing out caused by diarrhea, so it is
not unreasonable to assume that the absence of an appendix
may impact or delay recovery under similar circumstances.
Atresia
Atresia is a condition in which an orifice or passage
in the body in abnormally closed or absent. It may occur
at any of several points along the GI tract and each
are considered a congenital malformation or birth defect.
Some atresias can be detected before birth during routine
sonograms.
Esophageal atresia (EA) is a failure
of the esophagus to develop as a continuous passage,
ending instead in a blind pouch. it may occur together
with or separately from tracheoesophageal fistula (TEF),
an abnormal opening between the trachea and the esophagus.
EA is suspected in newborn infants with excessive drooling,
choking, coughing, and sneezing. Respiratory distress
is typical. If a catheter finds resistance or complete
blockage, then other tests or scans can confirm the
diagnosis. Oral feeding is stopped and intravenous fluids
are started. Surgery will usually repair the defect
and complete recovery and normal development are typical
with appropriate post-surgical care and feeding, but
babies with EA may have other congenital problems, sometimes
involving the heart, spine, or kidneys, which will require
diagnosis and treatment during infancy.
Biliary atresia is a rare condition
in newborn infants in which the common bile duct is
blocked or absent. If unrecognized or untreated, it
leads to liver failure, but its symptoms are initially
indistinguishable from neonatal jaundice (a more common
condition). If the intrahepatic biliary tree is unaffected,
surgery can reconstruct the defect (Kasai procedure).
If the atresia is complete, a liver transplant is necessary.
Intestinal atresia, including duodenal,
jejunal, ileal, and colon atresia, by location, may
show up in sonograms as dilated intestinal segments
due to the blockage, or by the build up of too much
amniotic fluid in the mother’s uterus. Some fetal
bowel obstructions are chromosomal in origin, and amniocentesis
may reveal these abnormalities. Fetal and neonatal intestinal
atresia are corrected surgically after birth, either
completely or by temporary use of a stoma.
Anal atresia, or imperforate anus,
is a congenital defect is which the rectum is malformed
or complete closed at the anus. There are several forms,
and as with most atresias, they are usually present
with other problems involving the heart, spine, or kidneys.
Surgical correction with or without a protective colostomy
is usually taken within the first day of life. Depending
on the location of the lesion, children with corrected
imperforate anus may or may not have problems with bowel
control, but will almost always have problems with constipation.
Further surgery later in life may adjust the angle between
the anus and rectum and/or remove an excessively dilated
section of colon, improving outcomes for both continence
and constipation.
Intussusception
Sometimes one section of the bowel tunnels into an adjoining
section, much like a collapsible telescope. Called intussusception,
this condition can occur in either the small intestine
or the large intestine, or between them. In infants
and children, intussusception is one of the most common
causes of intestinal obstruction.
Symptoms of intussusception can include:
-
bowel movements mixed with blood and mucus
- dehydration
- diarrhea
- distention
or lump in the abdomen
- fever
- intermittent
abdominal pain
- lethargy
- vomiting
bile
- shock
(e.g., low blood pressure, elevated pulse) requiring
immediate attention
Causes of intussusception in children are not fully
understood, but may include:
- viral
infection
- vaccine
for rotavirus (no longer given)
- malrotation
If intussusception is not diagnosed promptly, it can
cause serious damage to the affected portion of the
lower GI tract. A range of diagnostic tests can be applied
to identify the condition and possibly identify an underlying
cause. X-rays of the abdomen may suggest a bowel obstruction.
Upper and lower GI series can locate the telescoping.
CT scans may help. An air or barium enema may sometimes
correct the problem, if not the original cause, by pushing
the affected section of the bowel into proper position.
Surgery may be required to straighten or remove the
affection section. Outcome depends on the stage of intussusception
at diagnosis and the underlying cause, but with early
treatment the prognosis is generally excellent. In some
cases, intussusception in children may be temporary
and resolve spontaneously.
Malrotation and Volvulus
Sometimes during embryonic development, a section of
the lower GI tract may not rotate completely into its
final position. This condition is called malrotation.
If the cecum of the large intestine is not positioned
correctly, the bands of tissue that normally hold it
in place may cross over and block part of the small
intestine. If either the small or large intestines have
not rotated completely, the supporting mesentery may
be incompletely attached to the back of the abdominal
cavity, resulting in a mobile or floppy bowel prone
to twisting, a condition called volvulus (either cecal
or sigmoid, which is more common). Malrotation is associated
with other GI conditions, such as Hirschsprung’s
disease** and bowel atresia (above).
Symptoms of malrotation can include:
- vomiting
bile (especially in infants)
- abdominal
pain
- diarrhea
- constipation
- bloody
stool or rectal bleeding
- failure
to thrive
Causes of malrotation are unclear, but it is considered
a congenital abnormality. Most cases are identified
in infants, and most of those in the first month of
life.
Diagnosis and testing include x-rays to determine if
there is a blockage, upper GI series to locate the point
of obstruction, and lower GI series to determine the
position of the colon. CT scan may also be used.
Surgery to relieve blockage caused by malrotation is
generally successful and allows the digestive system
to function and develop normally.
Links
to key .org and .gov sites
National Digestive Diseases Information Clearinghouse
(NDDIFC)
digestive.niddk.nih.gov/ddiseases/a-z.asp
NDDIFC Easy-to-Read Publications
digestive.niddk.nih.gov/ddiseases/ez.asp
NDDIFC About Kids
aboutkidsgi.org/site/about-gi-health-in-kids
International Foundation for Functional Gastrointestinal
Disorders (IFFGD)
aboutgimotility.org
Pediatric Digital Library and Learning Collaboratory
pediatriceducation.org
General
Citations
Bruce M. Carlson (2004). Human Embryology and Developmental
Biology, 3rd edition, Saint Louis: Mosby
Complete Directory for Pediatric Disorders 2007/08 Edition,
Grey House Publishing
Dorland's Illustrated Medical Dictionary, 31st Edition
(2007)
Journal of Pediatric Gastroenterology & Nutrition.
Lippincott Williams & Wilkins
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