Motility in the
|Figure 34-8 Movement of intestinal contents
segmentation and peristalsis. A,
movements of food showing
squeeze the food back and forth,
mixing it with
enzymes. The sequential mixing movements
occur at about 1-second intervals.
movement, showing how food is
forward by a traveling wave of contraction.
Food is moved through the digestive
tract by cilia or by specialized musculature,
and often by both. Movement is
usually by cilia in the acoelomate and
pseudocoelomate metazoa that lack the
mesodermally derived gut musculature
of true coelomates. Cilia move intestinal
fluids and materials also in some
eucoelomates, such as most molluscs, in
which the coelom is weakly developed.
In animals with well-developed coeloms,
the gut is usually lined with
two opposing layers of smooth muscle: a
longitudinal layer, in which the smooth
muscle fibers run parallel with the length
of the gut, and a circular layer, in which
the muscle fibers embrace the circumference
of the gut. The most characteristic
gut movement is segmentation, the
alternate constriction of rings of smooth
muscle of the intestine that constantly
divide and squeeze the contents back
and forth (Figure 34-8A). Walter B. Cannon
of homeostasis fame, while
still a medical student at Harvard in 1900,
was the first to use X rays to watch segmentation
in experimental animals that
had been fed suspensions of barium sulfate.
Segmentation serves to mix food
but does not move it through the gut.
Another kind of muscular action, called
peristalsis, sweeps the food down the
gut with waves of contraction of circular
muscle (Figure 34-8B).