Human digestive system
Chewing, in which food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which can be swallowed down the esophagus to enter the stomach. Here it is mixed with gastric acid until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas. Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands. Digestion is helped by the chewing of food carried out by the muscles of mastication, by the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
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.