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.
Omasum
The omasum has two physiological compartments: omasal canal that transfers food from the reticulum to the omasum, and the inter-laminate recesses between the mucosal laminae which provide the area for absorption. The omasum is where food particles that are small enough get transferred into the abomasum for enzymatic digestion. In ruminants with a more sophisticated omasum, the large surface area allows it to play a key role in the absorption of water, electrolytes, volatile fatty acids, minerals, and the fermentation of food.