The rumen is the 'stomach' of cows, sheeps and goats ; it works like so: food goes in the mouth, chewed up and down the esophagus, goes into the rumen where bacteria start digesting the parts the animal can't, then it goes into the reticulum to be grinded up even more (whatever is too big is sent back up to be chewed again), then it goes into the omasum to be grinded up and then into the abomasum (very acidic) where enzymes act upon it until digested completely.