Deer are ruminants. This means that as the cow they have a digestive system which obliges them to chew, regurgitate and chew again their cud: they ruminate. They have the characteristic to be able to digest the cellulose (essential component of the plants), on contrary to the digestive system of the other animals (like us for exemple).
For that they have a stomach in 4 parts:
The pasture :
There is a first fast chewing which goes in the first stomach room ( the rumen ). This room is used as storage, the bolus is mixed in a bath of bacteria which attack cellulose. This fermentation releases the acids amino contents in cellulose and which thus can be assimilated by the organism. Fermentation produces also methane, a gas which the animal must evacuate very regularly.
Later, when the animal is at rest, the cud of regurgitation go up gently for a second and longer chewing which makes it possible to well break remaining cellulose fibres.
The whole go over then in the second stomach room ( the reticulum ), then the finest elements pass in the third stomach room ( the omasum ) for the last triturage and an absorption of 60 à.70.% of water. The elements largest contents in the reticulum set out again for a cycle of chewing.
Then the resulting cud finishes in the abomasum where the action of the gastric juices begins digestion itself. The continuous whole then its way in the intestine.