Chapter 23
The Digestive System
875
23
Te bile duct, delivering bile from the liver, and the main
pancreatic duct, carrying pancreatic juice from the pancreas,
unite in the wall of the duodenum in a bulblike point called the
hepatopancreatic ampulla
(hep
0
ah-to-pan
0
kre-at
9
ik am-pul
9
ah;
ampulla
5
flask). Te ampulla opens into the duodenum via
the volcano-shaped
major duodenal papilla
. A smooth muscle
valve called the
hepatopancreatic sphincter
controls the entry
of bile and pancreatic juice.
Te
jejunum
(jĕ-joo
9
num; “empty”), about 2.5 m (8 F) long,
extends from the duodenum to the ileum. Te
ileum
(il
9
e-um;
“twisted”), approximately 3.6 m (12 F) in length, joins the large
intestine at the ileocecal valve. Te jejunum and ileum hang in
sausagelike coils in the central and lower part of the abdominal
cavity, suspended from the posterior abdominal wall by the fan-
shaped
mesentery
(see ±igure 23.30). Te large intestine encir-
cles these more distal parts of the small intestine.
Nerve fibers serving the small intestine include parasym-
pathetics from the vagus and sympathetics from the thoracic
splanchnic nerves, both relayed through the superior mesen-
teric (and celiac) plexus.
Te arterial supply is primarily from the superior mesenteric
artery (pp. 732–733). Te veins parallel the arteries and typi-
cally drain into the superior mesenteric vein. ±rom there, the
nutrient-rich venous blood from the small intestine drains into
the hepatic portal vein, which carries it to the liver.
and pancreas (digestive enzymes). We will also consider these
accessory organs in this section.
The Small Intestine
Te
small intestine
is the body’s major digestive organ. Within
its twisted passageways, digestion is completed and virtually all
absorption occurs.
Gross Anatomy
Te small intestine is a convoluted tube extending from the py-
loric sphincter to the
ileocecal valve (sphincter)
(il
0
e-o-se
9
kal)
where it joins the large intestine. It is the longest part of the
alimentary canal, but is only about half the diameter of the large
intestine, ranging from 2.5 to 4 cm (1–1.6 inches). Although 6–7
m long (approximately 20 F, the height of a two-story building)
in a cadaver, the small intestine is only 2–4 m (7–13 F) long dur-
ing life because of muscle tone.
Te small intestine has three subdivisions: the duodenum,
which is mostly retroperitoneal, and the jejunum and ileum, both
intraperitoneal organs (see ±igure 23.1). Te relatively immovable
duodenum
(du
0
o-de
9
num; “twelve finger widths long”), which
curves around the head of the pancreas, is about 25 cm (10 inches)
long
(Figure 23.21)
. Although it is the shortest intestinal subdivi-
sion, the duodenum has the most features of interest.
Jejunum
Mucosa
with folds
Cystic duct
Duodenum
Hepatopancreatic
ampulla and sphincter
Gallbladder
Right and left
hepatic ducts
of liver
Bile duct and sphincter
Main pancreatic duct and sphincter
Pancreas
Tail of pancreas
Head of pancreas
Common hepatic duct
Major duodenal
papilla
Accessory pancreatic duct
Figure 23.21
The duodenum of the small intestine, and related organs.
Ducts from the
pancreas, gallbladder, and liver empty into the duodenum.
previous page 909 Human Anatomy and Physiology (9th ed ) 2012 read online next page 911 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off