The meal you ate today is outside the body even while it is traveling through your body. The alimentary canal is the pathway your food moves as it makes its way through the digestive system. That apple you placed in your mouth, chewed and swallowed, is technically outside the body even as it passes from mouth to esophagus, into the stomach, and through the intestine. Because the oral and anal ends interact directly with the environment, the canal itself is out rather than in. This quirk of anatomy explains why an animal, like the hippopotamus, is capable of living and swimming in its excrement. Often congregating in muddy pools and bays along lakes or rivers, the famed gaping mouth can be seen swallowing what appears to be the feces of its neighbor. No worries, as the hippo has a robust immune system, and is simply passing the contents from the outside, through its body that is anatomically not considered the inside.