The rumen is the site of fermentation. It has a fermentation chamber that can contain 30-60 gallons. The rumen includes both a ventral and dorsal sac. The sacs are made of stratified squamous epithelium with papillae. The rumen is responsible for the absorption of volatile fatty acids such as acetic, butyric, and propionic. 

Microbes within the rumen produce cellulase which breaks down complex cellulose that animals themselves are unable to digest. These microbes allow for the digestion of plant material and this fermentation creates methane and carbon dioxide.