A mucous membrane is a specialized epithelial tissue layer that lines body cavities and hollow organs opening to the exterior, including the oral cavity, nasal passages, gastrointestinal tract, and urogenital tract. The tissue consists of surface epithelium underlain by a lamina propria of loose connective tissue; goblet cells and submucosal glands secrete mucus that lubricates and protects the underlying tissue from mechanical abrasion and microbial invasion. In equine clinical examination, the color and moisture of oral mucous membranes are primary indicators of cardiovascular status and hydration. Healthy membranes are pink and moist with a capillary refill time of one to two seconds; pale, white, gray, or yellow membranes indicate anemia, shock, liver disease, or other systemic compromise. Assessment of mucous membranes is a key step in evaluating reading membrane color during a colic assessment cases and is included in routine wellness checks covered in the routine wellness checks. See also the mucus these membranes secrete.