A portion of a lamb's stomach, with a transverse section of the third stomach towards the rear of the preparation, photograph.
Ruminants such as sheep possess a four-chambered stomach to aid in the digestion of plants and grasses, a food source which vertebrates are unable to digest unaided. The reason for this is grasses consist of cellulose, a type of sugar. Vertebrates do not possess the enzyme cellulase which is required to break down cellulose.
During grazing, sheep partially chew grass and plants, mixing them into a bolus with saliva. The bolus is swallowed and travels to the rumen, the first of the four-chambers of the stomach (bottom of image). The rumen stores and processes the plant material, and can store up to 95 litres of undigested food. Importantly, it also contains billions of anaerobic cellulase-possessing bacteria, which are able to break down grasses and plants for the animal.
The partially digested food is then transported to the second stomach compartment, the reticulum, which can be identified due to its honeycomb appearance (top of image). The reticulum traps large food particles, which are regurgitated and re-chewed to form a new bolus (as in chewing the cud). The folds of the omasum, or third chamber, act as a water filter, ensuring that water is squeezed out of the feed, and remains in the rumen. Lastly, the abomasum is biologically most similar to our own stomach. It contains a low pH and enzymes to digest proteins into amino acids. These amino acids are then absorbed by the sheep in the small intestine.
