Transcript No. 6

No. 6
1. Small Intestine
2. Great Intestine
Section 5 The Small Intestine
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The small intestine—is a convoluted tube,
extending from the pylorus to the ileocecal valve,
where it joins the large intestine. It is the longest
part of the digestive tract, being 6～7 m.
Division: The small intestine can be divided into
three regions that are not otherwise distinct from
each other:
Duodenum: a short, curved section which is
devoid of a mesentery and is named the
duodenum.
Jejunum, and Ileum: the long, greatly coiled
part which is attached to the posterior abdominal
wall by the mesentery, and of which the proximal
2/5 constitutes the jejunum, and the distal 3/5
the ileum.
Ⅰ. The Duodenum
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It is the shortest, widest and most fixed part of
the small intestine, about 25 cm long. It has no
mesentery, and thus is only partially covered with
peritoneum.
Its course presents a remarkably constant curve,
somewhat of the shape of an incomplete circle,
which encloses the head of the pancreas.
It begins at the pylorus and ends opposite the
second lumbar vertebra in the jejunum.
For descriptive purpose it is divided into four
parts i.e.
the superior part,
the descending part,
the horizontal part,
the ascending part.
Ⅰ) The Superior Part (first part)
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It is about 3 cm long, and is the
most movable of the four parts.
It begins at the pylorus, and
continues with its descending part at
the area of the neck of the
gallbladder, where it forms the
superior duodenal flexure.
Ⅱ) The Descending Part (second
part)
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It is 8～10 cm long, descends from the superior
duodenal flexure, along the right side of the
vertebral column, and at the border of third
lumbar vertebra continues with its horizontal part,
where it forms the inferior duodenal flexure.
The common bile duct from the liver and the
pancreatic duct from the pancreas join together
to form the hepatopancreatic ampulla
(ampulla of Vater) , which empties into the
duodenum at the major duodenal papilla. This
opening is surrounded by a sphincter muscle
called the hepatopancreatic sphincter
(sphincter of Oddi) .
Ⅲ) The Horizontal Part (inferior or
third part)
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It is about 10 cm long, begins at the
inferior duodenal flexure.
Ⅳ) The Ascending Part (fourth
part)
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It is about 2.5 cm long, ascends to the
level of the upper border of the second
lumber vertebra, where it turns ventrally
at the duodenojejunal flexure and is
continuous with the jejunum.
The terminal part of the duodenum and
the duodenojejunal flexure are usually
described and fixed in position by the
suspensory muscle of duodenum
(suspensory muscle, or ligament of
Treitz).
Ⅱ. The Jejunum
The next 2.5 m or so of the small intestine is the
jejunum.
 This portion is suspended in the abdominal cavity
by a mesentery.
①It has a diameter of about 4 cm, and is thicker,
redder and more vascular than the ileum.
②The circular folds of its mucous membrane are
large and thickly set, and its villi surpass those of
the ileum in size.
③The aggregated lymphatic follicles are almost
absent in the upper part of the jejunum; in the
lower part they are fewer and smaller than those
in the ileum and tend to assume a circular form.
④The most part of jejunum lies in the umbilical
region, but it may extend into any of the
surrounding areas.
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Ⅲ. The Ileum
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The ileum is the remaining 3.5 m or
so of the small intestine. Like the
jejunum, the ileum, is suspended
from the posterior body wall by a
mesentery.
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① It has a diameter of 3.5 cm, and its wall is thinner than
that of the jejunum.
② A few circular folds are present in the upper part of the
ileum, but they are small and disappear almost entirely
towards its lower end.
③ the aggregated lymphatic follicles are, however, large
and more numerous than those in the jejunum.
④ For the most part the ileum is situated in the
hypogastric and pelvic regions. The lower portion of the
ileum usually lies in the pelvis.
It ends in the right iliac fossa by opening into the medial
side of the junction of the cecum and ascending colon.
The jejunum and ileum are attached to the posterior
abdominal wall by an extensive fold of peritoneum, termed
the mesentery, which allows of very free movement, so
that each coil can accommodate itself to changes in form
and position.
Section 6 The Large Intestine
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The large intestine, which is about
1.5 m long, 6.5 cm in diameter,
extends from the ileocecal valve to
the anus. Its certain parts are
attached to the posterior abdominal
wall by its mesocolon.
It is so named because its diameter
in most regions is greater than that
of the small intestine.
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The characteristics of the large intestine:
The large intestine differs considerably in
structure, appearance, size and
arrangement from the small intestine: It
has a greater caliber; for the most part, it
is more fixed in position.
The outstanding features are as the
follows, with the exceptions of the cecum,
the vermiform appendix and the rectum.
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1. colic band: Its longitudinal muscular fibers
form three colic bands.
2. haustras of colon: Since the colic bands are
shorter than the circular muscular coat, the colon
is puckered and sacculated, the sacculations
being known as haustras of colon.
3. epiploic appendices: Small, peritoneum—
covered, adipose projections, termed epiploic
appendices, are found scattered over the free
surface of the whole of the large intestine.
The large intestine is divided into several parts:
the cecum,
colon,
rectum,
anal canal.
Ⅰ. The Cecum
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The large intestine begins as a blind
pouch called the cecum, which
receives the ileum of the small
intestine.
It is about 6 cm long and 7.5 cm
wide. It is a blind pouch of the closed
end is directed downward, and it
opens above into the ascending colon.
Ⅱ. The Vermiform Appendix
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It is a narrow, worm shaped tube, which
springs from the posteromedial wall of the
cecum, 2 cm or less below the end of the
ileum. It is about 9 cm long and about 1.5
cm wide.
The appendix is variable in position.
Most commonly, the tip lies retrocecally or
hangs over the brim of the lesser pelvis.
The wall of the appendix contains
numerous lymphatic nodules.
Ⅲ. The Colon
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It may be considered in four parts:
the ascending,
transverse,
descending,
sigmoid.
The large intestine extends upward from
the cecum as the ascending colon.
The ascending colon is not supported by a
mesentery; instead, it lies tightly against
the posterior wall of the abdomen.
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Just beneath the liver, the ascending colon
bends sharply to the left (right colic
flexure) and crosses the abdominal cavity
as the transverse colon. This portion of
the colon is suspended by a mesentery
called the mesocolon.
In the vicinity of the spleen, the
transverse colon bends downward (left
colic flexure) and forms the descending
colon.
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The descending colon, like the ascending
colon, is retroperitoneal.
Where the descending colon reaches the
left pelvic brim, it curves to the midplane
via an S-shaped sigmoid colon.
The sigmoid colon is closely surrounded by
peritoneum, which forms a mesentery, the
sigmoid mesocolon.
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The position and shape of the sigmoid
colon vary very much, and depend on:
① its length,
② the length and freedom of its
mesocolon,
③ the condition of distension,
④ the condition of the rectum and bladder.
Usually it is relatively free within the
lesser pelvis below the small intestine
Ⅳ. The Rectum
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It is continuous with the sigmoid
colon at the level of the third sacral
vertebra. It is about 12 cm long and
its upper part has the same diameter
as the sigmoid colon (about 4 cm in
the empty state), but its lower part
of the rectum is dilated to form the
ampulla of rectum.
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Sacral flexure and perineal flexure of the
rectum:
From its origin it descends, following the
concavity of the sacrum and coccyx,
forming an anteroposterior curve known
as the sacral flexure of the rectum. It
thus passes at first downwards and
backwards, then downwards, and finally
downwards and forwards to become
continuous with anal canal by passing
through the pelvic diaphragm.
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The anorectal junction is situated 2～
3 cm in front of and slightly below
the tip of the coccyx. From this level,
which in the male is opposite the
apex of the prostate, the anal canal
passes downwards and backwards
from the lower end of the rectum,
the backward bend of the gut at the
anorectal junction being termed the
perineal flexure of the rectum.
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The peritoneum covers the upper 1/3 of
the rectum on its front and sides, the
middle third on its front only, and does not
cover anywhere of the lower third.
In the empty state of the rectum, the
mucous membrane of its lower part
presents a number of longitudinal folds
which are effaced by distension of the
rectum. Besides, there are three
permanent transverse folds of rectum
(semilunar rectal folds).
Ⅴ. The Anal Canal
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The terminal 3 to 4 cm of the large
intestine is called the anal canal. This
region is located below the pelvic
diaphragm and thus is outside the
pelvis.
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Morphology of the anal canal:
In the lower part of the canal the mucous
membrane present 6～10 vertical folds,
the anal columns.
The lower ends of these columns are
joined together by small crescentic valvelike folds of mucous membrane, the anal
valves, above each of which lies a small
recess or anal sinuses.
The line along which the anal valves are
situated is termed the dentate line.
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The succeeding part of the anal canal
extends for about 1 cm below the anal
valves, and is known as the anal pecten.
Below the lower border of the anal pecten,
there is the white line, it is the
transitional zone between the anal mucous
membrane and the anal skin. It can be felt
in the digital examination of the anal canal
because it lies at the interval between the
sphincter ani internus and the
sphincter ani externus.
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The walls of the anal canal are surrounded
by a complex of muscular sphincters,
which can be divided into internal and
external parts. At the anorectal junction
the circular muscle coat of the rectum
becomes considerably thickened (5-8 mm)
to form the sphincter ani internus. The
sphincter ani externus surrounds the
whole length of the anal canal; it is
usually described as consisting of three
parts and composed of striated muscle.