Rumen

The rumen is the fermentation vat. It is the largest part of the stomach. Its internal surface is covered with projections called papillae, which provide a greater surface area for absorption as well as supporting the mechanical movement of food around the rumen. It contains a large number of microbes – bacteria that digest the cellulose in the plant material. This releases the sugars and nutrients within the plant cells. The microbes that pass into the abomasum and small intestine are digested and provide a source of protein for the cows. The rumen and reticulum together can hold 50–120 litres of food and fluid. 

IMAGE: Dissection highlighting the rumen in a sheep, Dr Sarah Pain, Massey University.