Describe the appearance of the inside of the stomach. how do the rugae within the stomach aid in mechanical digestion?

When the stomach is empty, the walls are folded into rugae (stomach folds), which allows the stomach to expand as more food fills it. 
In the stomach, food undergoes chemical and mechanical digestion. The peristaltic contractions churn the bolus, which mixes with strong digestive juices that the stomach lining secrete. The stomach walls contain three layers of smooth muscle arranged in longitudinal, circular, and oblique rows. These muscles allow the stomach to squeeze and churn the food during mechanical digestion.