Final answer:
Circular folds, also known as plica circulares, are the circumferential folds that encircle the digestive tract and facilitate absorption by slowing down the movement of chyme in the small intestine.
Step-by-step explanation:
The circumferential folds that encircle the digestive tract are known as circle folds or plica circulares. These are deep ridges in the mucosa and submucosa, which start near the proximal part of the duodenum and end near the middle of the ileum. These circle folds play a crucial role in the digestive system as they facilitate absorption by causing the chyme to spiral through the small intestine. This spiraling action slows down the chyme movement, which gives more time for the nutrients to be fully absorbed, thereby increasing the efficiency of the digestive process. The circular folds are different from adventitia, rugae, muscularis externa, and serosa, which are other structures of the alimentary canal.