Final answer:
The stomach requires three layers of muscle to vigorously churn and mix food, aiding in mechanical digestion and enabling peristalsis, which further helps in mixing the food with digestive juices and moving it into the small intestine.
Step-by-step explanation:
The stomach needs three layers of muscle in the alimentary canal to effectively churn and mix the food as part of the digestive process. The muscularis externa, commonly known as the muscularis, consists of three smooth muscle layers in the stomach: an inner oblique layer, a middle circular layer, and an outer longitudinal layer. This extra layer, the inner oblique muscle, provides the stomach with the ability to vigorously churn and mix food. This mechanical digestion is critical for breaking down food into smaller particles to facilitate chemical digestion and absorption later in the digestive tract.
Additionally, the muscular walls of the stomach perform waves of muscle contractions, known as peristalsis, which not only mix the food with digestive secretions like gastric acid and enzymes essential for protein and fat digestion but also help to propel the food toward the small intestine. This complexity in the muscular structure allows the stomach to function effectively in its role as a primary organ for digestion.