This slide is a cross section from the small intestine. Projecting into the intestinal lumen (space) are numerous finger-like projections called villi, which function to slow the passage of food and increase the surface area for the absorption of nutrients. The lining of these villi is a tissue layer called the mucosa, which is made up of simple columnar epithelial cells. Interspersed among these columnar cells are goblet cells that secrete mucus into the lumen of the intestine. During routine histological preparation, the mucus is lost, leaving a clear or lightly stained cytoplasm. Beneath a thin, outer covering of the intestine called the serosa is a thick layer of smooth muscle cells called the muscularis externa. The muscularis externa is divided into an outer longitudinal muscle layer with cells that run along the axis of the intestine and an inner, circular muscle layer whose fibers encircle the organ. Peristaltic contraction of these two muscle layers keeps food moving through the digestive tract.