Final answer:
Gastrin is the hormone secreted by the stomach that controls muscular contraction and secretions, promoting the secretion of gastric acid for protein digestion.
Step-by-step explanation:
The hormone secreted by the stomach that controls muscular contraction and secretions of the stomach is gastrin. Gastrin is produced by G cells in the stomach in response to the presence of food, specifically proteins. It plays a crucial role in the digestive process by stimulating the secretion of gastric acid (hydrochloric acid) from the parietal cells of the stomach mucosa, thus aiding in protein digestion. When the stomach is emptied, another hormone called somatostatin acts to inhibit the release of hydrochloric acid, as part of a negative feedback mechanism to control the stomach's acidic environment.