The mucosa consists of mucosal glands, crypts, and other stuff
intussusception
Lumen
It is the space within the intestine
lumanate its inside
small intestine
The word is lumen. It means the interior of a hollow tubular structure such as the intestine.
The lumen is the inner open space or cavity of a tubular organ, such as a blood vessel or an intestine
Submucosa
Troposphere is the lowest layer of the atmosphere, which is adjacent to the earth's surface.
The small intestine has many villi, small projections into the lumen of the small intestine. These villi increase the surface area of the small intestine, and increase the funcitonal area for the absorbtion of nutrients.
Villi
muscular tissue (specifically, smooth muscle)