The lumen.
The lumen is a hollow space or tube within an organ in which substances are often carried or transported. The lumen of the small intestine is the opening that connects the stomach to the colon, which allows substances to pass through the small intestine and be absorbed into the body.
The lumen is lined with a simple columnar epithelial lining that consists of a single layer of cells that are lined up side by side forming a flat surface. The lumen also contains secretions from the pancreas and gallbladder, which help digest food in the small intestine.