The wall of small intestine consists of four layers. They are: outer serous (peritoneal) layer, next muscular layer with longitudinal and circular fibres, then there is submucous layer and muscularis mucosae and the inner most is mucous layer. The mucosa contains numerous folds in it, which projects towards the lumen called as plicae circulares. There are further minute thin finger like processes called as villi present over the folds of plicae circulares. In addition to these two folds the membrane of cells facing towards lumen is having ultra-microscopic finger like projections towards the lumen called microvilli.medical billing training online trains you all these.