Internal anal sphincters are smooth muscle and are involuntary, external anal sphincters are skeletal muscle and are voluntary. Functions to start enzymatic catabolism of complex starches and other carbohydrates into smaller carbohydrates and disaccharides.
Amylase is a carbohydrase.
Cells use CO2 from metabolism and water to combine the two, using carbonic anhydrase, to form carbonic acid H2CO3. The hydrogen ion is actively transported across the cell membrane to the stomach lumen. Stomach lumen attains a pH of 2. Were the stomach to produce active pepsin in its cytoplasm, the cell would be harmed. Pepsin is a protease and acts to chemically digest proteins to amino acids.