and serosa (Figure 14.2b). These layers include epithelial
tissue (to help absorb food), connective tissue (for sup-
port), and muscle (to propel the food on its way).
The GI tract has a rich blood supply for absorbing
nutrients and networks of neurons. The enteric ner-
vous system (ENS), the “brain of the gut,” coordinates
its movements and secretions. ENS neurons within the
submucosa control the secretions of the organs of the
GI tract, and ENS neurons within the muscularis con-
trol the frequency and strength of its contractions. The
ENS is subject to regulation by the autonomic nervous
system (ANS). You will learn more about the neural
control of digestion later in this chapter.
Let’s take a closer look at the digestive system by fol-
lowing your breakfast of waffles, sausages, and eggs on its
b. The four layers of the gastrointestinal tract: serosa, muscularis, submucosa, and mucosa
—Connective tissue that binds
mucosa to muscularis:
• Neuronal networks from enteric nervous
system (ENS) and autonomic nervous
system (ANS) control secretions into the
• Blood and lymphatic vessels absorb food
• Glands in submucosa secrete substances
into GI tract.
Duct from gland
outside GI tract
—Inner lining of digestive tract:
Lam ina propria
is connective tissue
M uscularis m ucosae
is a thin layer
of smooth muscle that creates folds in
the mucosa, which increases
absorptive surface area.
(plexus of Meissner)
(plexus of Auerbach)
—Thick layer of muscle
(mostly smooth muscle but part skeletal
in esophagus and anal sphincter
muscles) that break down food, mix it
and move it through the GI tract:
• Inner sheet of circular muscle.
• Outer layer of longitudinal muscle
• ENS neurons control frequency and
strength of muscle contractions.
—Outermost layer, which
consists of the following:
• Connective tissue
The serosa secretes a watery fluid
that lubricates the GI tract, allowing it
to slide against other organs.
Abdominal fat is located in which of the following?
b. Greater omentum
Let's Journey Through the Digestive System 403