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CHAPTER SEVENTEEN
Digestive System
muscle (muscularis mucosae). In some regions, the
mucosa is folded with tiny projections that extend
into the passageway, or
lumen,
of the digestive tube;
the folds increase the absorptive surface area. The
mucosa also has glands that are tubular invaginations
into which the lining cells secrete mucus and digestive
enzymes. The mucosa protects the tissues beneath it
and carries on secretion and absorption.
Although the four-layered structure persists throughout the
alimentary canal, certain regions are specialized for partic-
ular functions. Beginning with the innermost tissues, these
layers, shown in
f gure 17.3
, include the following:
1.
Mucosa
(mu-ko
sah), or
mucous membrane.
This layer
is formed of surface epithelium, underlying connective
tissue (lamina propria), and a small amount of smooth
ALIMENTARY CANAL
Mouth
Mechanical breakdown
of food; begins chemical
digestion of carbohydrates
Pharynx
Connects mouth with
esophagus
Esophagus
Peristalsis pushes
food to stomach
Stomach
Secretes acid and
enzymes; mixes food
with secretions to
begin enzymatic
digestion of proteins
Rectum
Regulates elimination
of feces
Anus
Large intestine
Absorbs water and
electrolytes to form feces
Small intestine
Mixes food with bile
and pancreatic juice;
final enzymatic breakdown
of food molecules;
main site of
nutrient absorption
Salivary glands
Secrete saliva, which contains
enzymes that initiate breakdown
of carbohydrates
ACCESSORY ORGANS
Liver
Produces bile, which
emulsifies fat
Gallbladder
Stores bile and
introduces it into
small intestine
Pancreas
Produces and secretes
pancreatic juice, containing
digestive enzymes and
bicarbonate ions,
into small intestine
FIGURE 17.1
Organs of the digestive system.
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