Digestion
Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
Methanogens in digestive tract of ruminants
Methanogens are a group of microorganisms that can produce methane as a byproduct of their metabolism.They hold an important place in the digestive system of ruminants. The digestive tract of ruminants contain four major parts, they are abomasum, rumen, omasum and reticulum.The food with saliva is first passed to the rumen for breaking them into smaller particles and then it moves to the reticulum where the food is broken into further smaller particles and the indigestable particles are sent back for rechewing and then to rumen.  The majority of the anaerobic microbes assisting the cellulose breakdown occupy the rumen. They initiate the fermentation process.The animal observes the fatty acids, vitamins and nutrient content on passing the partially digested food from rumen to omasum which, decreases the pH level and thus initiates the release of enzymes for further break down the food which is later passed to the abomasumthat absorbs remaining nutrients before excretion.This process takes about 9–12 hours.