What is sandwiched between the layer of circular and longitudinal muscle in the muscularis externa?

Answer: The Auerbach plexus or myenteric plexus.

Explanation: The muscularis externa is a composed layer of muscles, located throughout the gastrointestinal tract. It has an inner circular muscular layer and a longitudinal external muscular layer. Between the two layers there is a part of the nervous system called Myenteric plexus.

A plexus is like a web of axons (nervous tissue) that provides a specific function. In this case, the myenteric plexus has a main function: motility. All the digestive tract (esophagus, stomach and intestine) possess this structure, to give motility or "peristalsis", so the food could travel all the way from the esophagus through the colon.