any structural anomaly of the distinct rodent non-glandular region of the stomach which is demarcated from the glandular stomach by the limiting ridge (margo plicatus); the forestomach accounts for about 60% (rat) and 70% (mouse) of the stomach volume and receives the esophagus at the cardiac antrum; the wall of the forestomach consists of, from inside to outside, stratified squamous epithelium, the lamina propria (epithelium and lamina propria form the mucosa), the muscularis mucosae, the submucosa, the muscularis interna and externa (inner circular and outer longitudinal layers of smooth muscle), and the serosa