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1. Small intestine
•The small intestine is the longest part of the alimentary canal, and the si
te of most enzymatic digestion and virtually all absorption of nutrients.
•Most digestive enzymes that operate within the small intestine are secre
ted not by
the small intestine, but by the pancreas.
•During digestion, the small intestine undergoes active segmentation mo
vements,
shuffling the chyme back andforth and maximizing its contact with the
nutrient-absorbing mucosa. Peristalsis propels chyme through the small i
ntestine in
about 3-6 hours.
•The small intestine has three subdivisions: the duodenum, jejunum and i
leum.
Although similar in general structure, each region has its characteristic
features.

2. Duodenum
•The duodenum is the
shortest, widest and
most fixed part of the
small intestine.
•It is so named because
its length
(approximately 25 cm)
•It extends from the
pylorus to the
duodenojejunal flexure.
•It is C-shaped around
the head of the
pancreas.

3. •The superior part passes upwards, backwards and laterally to the right side of
the 1st lumbar vertebra above the head of the pancreas.
•Posterior to it are the common bile duct and the inferior vena cava.
Superior
Part

4. •The
descending
part turns
down along
the right
side of the
2nd and
3rd lumbar
vertebrae.
•Posterior to it are the right renal vessels, upper part of the right ureter and a
variable amount of the right kidney.
Descending
Part

5. The hepatopancreatic ampulla (ampulla of Vater) (formed by the union of the
terminations of the bile and pancreatic ducts) opens on the summit of the major
duodenal papilla, which is on the internal surface of the posteromedial aspect of
the descending part.
There may be a smaller opening about 2cm above the major duodenal papilla;
this is the opening of the accessory pancreatic duct and it is called minor
duodenal papilla.

6. •The horizontal part crosses 3rd lumbar vertebra from right to left to become
continuous with the fourth part.
•The inferior vena cava, aorta, testicular or ovarian vessels, and the origin of the
inferior mesenteric artery are situated posterior to it.
Horizontal
Part

7. •The duodenojejunal flexure is fixed by a fibromuscular band, Suspe
nsory muscle of duodenum .
• It passes from the right crus of the diaphragm at the right side of th
e esophagus, behind the pancreas, and is attached to the duodenojeju
nal flexure posteriorly.
•The ascending
part turns
upwards and
ends at the
duodenojejunal
flexure at the
level of the 2n
d
lumbar verteb
ra.
Ascending
Part

8. • Suspensory muscle of duodenum (ligament of Treitz), a surgic
al landmark, descends from the right crus of diaphragm to duodenal ter
mination.

9. Jejunum & ileum
•The jejunum and ileum form
sausage-like coils that hang
from the posterior abdominal
wall by a mesentery, which
permits the coils to move
freely and are framed by the
large intestine.
•The jejunum makes up the
superior left part of this
coiled intestinal mass.
•The ileum makes up the
inferior right part of this
coiled intestinal mass. In
addition, some coils of the
ileum lie in the pelvis
between the bladder and
rectum or the uterus and
rectum.

10. Jejunum and ileum
Characteristic Jejunum Ileum
Position Upper 2/5, upper left pa
rt of abdominal cavity
Lower 3/5, lower right p
art of abdominal cavity
Diameter Greater Less
Wall Thicker Thin
Circular folds Larger, numerous and l
arge villi
Fewer，smaller and les
s abundant villi
Vascularity Greater Less
Color Deeper red Paler pink
Lymphatic follicles Solitary Aggregated
Fat in mesentery Less More

11. Large intestine
•The large intestine frames the small intestine on 3½ sides, forming an open
rectangle.
•The large has the following subdivisions: cecum, vermiform appendix, colon,
rectum, and anal canal.
•The colon is further divided into ascending, transverse, descending, and sigmoid
colon.

12. •Over most of its length, the large intestine exhibits three special features.
•The teniae coli (colic bands) are three longitudinal strips, spaced at equal
intervals on the surfaces of the cecum and colon. They are thickenings of the
longitudinal layer of muscularis.
•Because the teniae coli maintain muscle tone, they cause the large intestine to
pucker into many sacs called the haustra of colon, which make the colon a
typical sacculated appearance.
•Attached to the teniae coli are many small pieces of fat covered by visceral
peritoneum called the epiploic appendices. They vary greatly in size in different
parts of the large intestine, and are often rudimentary or absent in the cecum,
rectum and appendix.

13.  The cecum is situated in the right lower quadrant, where it lies in t
he iliac fossa above the lateral half of the inguinal ligament.
 The caecum in most cases is completely covered with peritoneum a
nd is therefore freely movable. But the posterior surface may be de
void of peritoneum so that it is fixed to the posterior abdominal w
all.
Cecum

14. •When the cecum is
distended, the
anterior surface
comes into contact
with the anterior
abdominal wall
and may be
palpated in the
living subject.
•When empty, its
anterior surface is
covered by coils
of the small
intestine.
•The medial surfac
is also related to
coils of small
intestine.

15. •The ileocecal orifice
is a transverse slit in
the posteromedial
wall of the cecum.
•This orifice is
guarded by a valve
composed of upper
and lower folds
which project into
the cecal lumen.
These folds form the
ileocecal valve.
•The ileocecal valve
acts as a sphincter
and relaxes at
frequent intervals to allow a small amount of the ileal contents to pass through.
The sphincter also prevents regurgitation of the cecal contents into the ileum,
but not very effectively.
•The orifice of the appendix, situated about 2 cm below the ileocecal orifice, is
overlapped by a fold of mucous membrane which does not function as a valve
to prevent the passage of the cecal contents into the appendix.

16. •The vermiform appendix is a worm-shaped blind tube.
•Its base is attached to the cecum inferior to the ileocecal junction. Its apex is free.
•The appendix varies in length from 2-23 cm, with a normal range of 7-12 cm.
•Normally the appendix has a complete peritoneal covering; the proximal half is
attached by mesentery, the mesoappendix; the distal half hangs free into the
peritoneal cavity.
Vermiform
appendix

17. The position of the appendix varies
considerably in different
individuals and in the same
individual from time to time.
It may lie (i) in the retrocaecal
fossa (the commonest position):
(ii) entirely within the pelvis
amongst the coils of the small
intestine; (iii) close to the
inguinal ligament; (iv) in front
of or behind the terminal part
of the ileum; or (v) to the lateral
side of the cecum.
When in the iliac fossa it is related
to the iliacus and psoas major,
and to the femoral nerve.

18. •Whatever the position
of the tip of the
appendix, the
position of the base
of the appendix
normally lies deep to
McBurney's point,
which is at the
junction of the lower
and middle thirds of
a line joining the
right anterior
superior iliac spine
and the umbilicus.

19. Colon
•The colon has several distinct segments.
•The ascending colon ascends along the right side of the posterior abd
ominal wall from the cecum, and reaches the level of the right kidney,
where it makes a right-angle turn, forming the right colic flexure.
•The ascending colon usually has no mesentery and is therefore relativ
ely fixed. The upper part of the ascending colon is covered in front by
coils of small intestine, but its lower part may come into direct contac
t with the anterior abdominal wall.

20. •The transverse colon extends to th
e left from the right colic flexure acr
oss the peritoneal cavity. Directly an
terior to the spleen, it bends acutely
downward, forming the left colic fle
xure.
•The transverse colon is suspended b
y the transverse mesocolon which pe
rmits movement.
•The left colic flexure is higher than
the right and is in contact with the sp
leen, the greater curvature of the sto
mach, the tail of the pancreas and the
anterior surface of the left kidney.

21. •The descending colon descends
from the left colic flexure along
the left side of the posterior abdo
minal wall. After crossing the lef
t iliac fossa, it becomes
continuous with the sigmoid col
on at the brim of the true pelvis.
•Its upper part is placed deeply a
nd is separated from the anterior
abdominal wall
by the termination of the transv
erse colon and coils of the small
intestine. When
distended, its lower part freque
ntly comes into contact with the
anterior abdominal wall above t
h inguinal ligament.

22. •Inferiorly, the colon enters th
e true pelvis as S-shaped sigm
oid colon. At the
level of the third sacral verte
bra, it continues to the rectum.
•The sigmoid colon is suspen
ded by the sigmoid mesocolo
n.
•The sigmoid colon lies deep i
n the pelvis with coils of the s
mall intestine separating it fr
om the bladder in the male an
d the uterus in the female.

23. The rectum is
continuous
proximally with
the sigmoid
colon at the
level of 3rd
sacral vertebra,
and at the level
of the tip of the
coccyx it bends
sharply
backwards to
become
continuous with
the anal canal.
Rectum

24. •Even though the word rectum
means “straight,” the rectum
actually has several tight bends.
•When viewed from anterior aspect, it is S-shaped and has three lateral
curves. The upper and lower curves are usually convex to the right and
the middle curve is convex to the left.

25. •These curves are represented
as three transverse folds of
the rectum, which are
formed by the mucous
membrane, submucosa and
circular muscle layer of the
rectal wall.
•The most constant transverse
fold of the rectum is middle
one situated anteriorly and
to the right just below the
level of the reflection of the
peritoneum from the rectum
to the bladder or vagina.
•These folds can be recognized
on examination of the rectum
in the living subject with a
proctoscope, and the main
fold may be felt on digital
examination.

26. •These transverse
folds prevent feces
from being passed
along with flatus
(gas).
•The dilated lower
part of the rectum
is called the rectal
ampulla, which
supports and
holds the fecal
mass before
defecation.
•Normally,
distension of the
rectum arouses
the desire to
defecate.

27. The relations of the rectum
are important since many
of the adjacent structures
can be palpated from its
lumen by an examining
finger.
Posteriorly, the rectum is
related to the lower three
sacral vertebrae and
coccyx.
The anterior relations of
the rectum differ in the
two sexes.
In the male, the rectum is
related to the seminal
vesicles, the terminations
of the ductus deferens,
urinary bladder and
prostate gland. Of these
structures the prostate gland can readily be felt by digital examinatio
n in the living subject.

28. •In the female,
the rectum is
related to the
uterus and
vagina. The
cervix of the
uterus is
readily
palpable as
a rounded
knob
projecting
backwards.
The ovaries
lie
anterolateral
to the rectum
and may
sometimes be
felt by digital examination.

29. •The anal canal is the
terminal part of the
large intestine.
•About 3 cm long, it
begins at the level of
the tip of the coccyx,
where the rectum
passes through the
levator ani. As the
levator ani form the
pelvic floor, thus, the
anal canal lies
entirely external to the
abdominopelvic cavity.
•It passes downwards and backwards and ends at the anus.
•Laterally the canal is separated from the fat of the ischiorectal fossa by the
levator ani and the external sphincter. The presence of fat in the fossa allows
the canal to dilate during the passage of fecal mass.
Anal canal

30. •The lining membrane of
the anal canal forms
six to ten vertical folds
called the anal columns.
•The lower ends of the
anal columns are joined
together by pocketlike
folds of mucous
membrane, the anal
valves.
•The space between an
anal valve and the anal
wall is called an anal
sinus.
•The function of the anal
columns and valves is
not known.
Interior of
anal canal

31. •The inferior comb-shaped
limit of the valves forms an
irregular line known as the
pectinate (dentate) line.
•This line indicates the
junction of the superior part
of the anal canal (derived
from the hindgut) and the
inferior part (derived from
the proctodeum).
•Because the mucosa superior
to this line is innervated by
visceral sensory fibers, it is
relatively insensitive to pain.
Inferior to the pectinate line,
however, the mucosa is
sensitive to pain because it is
innervated by somatic nerves.

32. •The anal canal is surrounded by an internal an
al sphincter of smooth muscle and an external a
nal sphincter of skeletal muscle.
•The internal anal sphincter contracts involunta
rily, whereas the external anal
sphincter contracts voluntarily to inhibit
defecation, both to prevent feces from leakin
g from the anus between defecations and to inh
ibit defecation during emotional stress.
•Kids learn to control the external anal sphincte
r during toilet training.

33. •The internal anal sphincter is a
thickening of the circular
muscle of the gut wall.
•The external anal sphincter is
usually described as tripartite,
but there is no very clear
distinction between the three
parts. The subcutaneous part
is deep to skin and does not
affect anal continence. The
superficial part is attached
posteriorly to the coccyx. It
encircles the anal canal to be
inserted into the central
tendon of the perineum.
The deep part has no bony
attachment and encircles the
upper part of the internal anal sphinc

34. The tonic contractions of the external and internal anal
sphincters keep the anus and anal canal closed, and are inhibited
during defecation. The external anal sphincter is stronger than
the internal anal aphincter, which appears to be unimportant for
normal fecal continence since complete surgical division of the
internal anal sphincter does not result in incontinence. If the
external anal sphincter is paralysed, however, sphincter control
is lost.
In addition to the sphincter, the lower part of the rectum and
the upper part of the anal canal are supported by the
puborectalis, which passes around their lateral and posterior
sides like a sling. Contraction of the puborectalis causes the angle
between the rectum and anal canal to become more acute. Thus,
its contraction is an important factor in preventing passage of
feces from the rectum to the anal canal.