Final answer:
The inner circular muscle layer of the GI tract is greatly thickened to form a sphincter at specific locations.
Step-by-step explanation:
The inner circular muscle layer of the gastrointestinal (GI) tract is greatly thickened to form a sphincter at specific locations along the tract. A sphincter is a ring-like muscle that contracts to close an opening or passage in the body. In the case of the GI tract, sphincters help control the flow and movement of food and waste through the different sections of the digestive system.