What structure is primarily involved in digestion but is not part of the small intestine?

The stomach is primarily involved in digestion but is not part of the small intestine. Its main function is to break down food through mechanical and chemical processes before it moves into the small intestine. The stomach uses gastric juices, which contain acids and enzymes, to initiate the digestion of proteins and to further liquefy the food. This allows for an efficient transition of partially digested food, known as chyme, into the small intestine, where the majority of nutrient absorption occurs.

While the pancreas, liver, and gallbladder also play crucial roles in digestion, they are not the primary site where the digestive process begins. The pancreas produces digestive enzymes and hormones, the liver generates bile which aids in fat digestion, and the gallbladder stores bile until it is needed. However, none of these structures are responsible for the initial breakdown of food in the same manner as the stomach.