654
UNIT FIVE
2.
Submucosa
(sub
mu-ko
sah). The submucosa contains
considerable loose connective tissue as well as glands,
blood vessels, lymphatic vessels, and nerves. Its
vessels nourish the surrounding tissues and carry away
absorbed materials.
3.
Muscular layer.
This layer, which provides movements
of the tube, consists of two coats of smooth muscle tissue.
The F bers of the inner coat encircle the tube. When
these
circular f
bers
(they are closed spirals) contract, the
diameter of the tube decreases. The F bers of the outer
muscular coat run lengthwise. When these
longitudinal
f
bers
(open spirals) contract, the tube shortens.
4.
Serosa
(se-r-o
sah), or
serous layer.
The serous layer,
or outer covering of the tube, is composed of the
visceral
peritoneum,
formed of epithelium on the outside and
connective tissue beneath. The cells of the serosa
protect underlying tissues and secrete serous fl
uid,
which moistens and lubricates the tube’s outer surface
so that the organs in the abdominal cavity slide freely
against one another.
Table 17.1
summarizes the characteristics of the layers of
the alimentary canal.
Movements of the Tube
The motor functions of the alimentary canal are of two
basic types—
mixing movements
and
propelling movements.
Mixing occurs when smooth muscles in small segments of
the tube contract rhythmically. ±or example, when the stom-
ach is full, waves of muscular contractions move along its
wall from one end to the other
(f g. 17.4
a
)
. These waves
occur every twenty seconds or so, and they mix foods with
the digestive juices that the mucosa secretes. In the small
intestine,
segmentation
is a type of movement that aids
mixing by alternately contracting and relaxing the smooth
muscle in nonadjacent segments of the organ (F g. 17.4
b
).
Segmentation does not follow a set pattern, so materials are
not moved along the tract in one direction.
Propelling movements include a wavelike motion called
peristalsis
(per
ı˘-stal
sis), in which a ring of contraction
occurs in the wall of the tube (F g. 17.4
c
). At the same time, the
muscular wall just ahead of the ring relaxes—a phenomenon
called
receptive relaxation.
As the wave moves along the tube,
it pushes the contents of the tube ahead of it. Peristalsis begins
when food expands the tube. It causes the sounds that can be
heard through a stethoscope applied to the abdominal wall.
A “GI camera” the size of a large vitamin pill can image the alimentary
canal, revealing blockages and sites of bleeding. The patient swallows
the capsule, which contains a camera, a light source, radio transmit-
ter, and batteries. Peristalsis moves it along, and about six hours after
swallowing, it transmits images from the small intestine to a device
worn on the physician’s belt. The information goes to a computer,
and still or video images are downloaded. The device, which is dis-
posable, leaves the body in the feces within a day or two.
Tongue
.5 meter (from tongue
to duodenum)
5.5 – 6.0 meters
(small intestine)
1.5 meters
(large intestine)
Esophagus
Stomach
Gallbladder
Pancreas
Duodenum
Jejunum
(2.2 – 2.4 m)
Ileum
(3.3 – 3.6 m)
Cecum
Appendix
Large
intestine
Anus
1.0 m
FIGURE 17.2
The alimentary canal is a muscular tube about 8
meters long.
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