The myenteric plexus is a network of nerve fibers and neuron cell
bodies that is tucked in among the interstices of the fibers in the
tunica muscularis. The plexus is an important component of the entire digestive tract. It's found not only in the stomach, but in the esophagus and intestines as well.
There is only one myenteric plexus; the casual use of the plural in some texts implies that there are several, but it's really all one network.
The smooth muscles of the tunica muscularis receive "orders" via these nerve fibers and contract in response. The plexus coordinates the contractions so that the appropriate kneading and squishing of the ingested food takes place. Here you see a knot of neurons, comprising one of the local nodes of the network. These are, of course, ganglia by definition: collections of neuron cell bodies outside the CNS.
The digestive tract thus has what amounts to its own division of the peripheral nervous system, sometimes called the "enteric nervous system," to distinguish it from the somatic and autonomic divisions. It is equivalent in complexity and autonomy to the others. Gastrointestinal sensory and motor activity pretty much runs on "autopilot" except when there is some overwhelming need for the CNS to get involved.
Monkey stomach; H&E stain, 1.5 µm plastic section, 400x
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