You have a hole running clear through your body. I mentioned as much on the 8 Reasons Why Your Body Is So Gross podcast episode. On one end of this hole is your anus; on the other end is your mouth. It would be surprising if this were news to you, of course. You've been eating and pooping your entire life and have surely gained at least a cursory awareness of the whole messy business over the years. While you may have grown comfortable with the idea that you have a hole running through you, perhaps you've not yet considered yourself as a doughnut.
That opening you have running through you turns you, geometrically speaking, from a solid body into a torus, a three-dimensional object featuring a single hole. And what is perhaps most interesting about this hole is that, just like the skin of our arms or face, it is where our body butts up against the outside world. That hole running through us: our throat, our stomach, our guts, our rectum -- what we typically think of as the most internally-located regions of our physical selves -- is actually outside our body.
It is a pretty amazing stretch of the outside world that runs through the center of our bodies. It features folds, valves and sphincters capable of containing liters of hydrochloric acid, mucous and all sorts of other juices and viscous liquids. Were it cut away from the inside of your body and stretched out, it would measure about 27 feet long (around 8 or so meters). Were you some sort of sicko and you cut this removed digestive system lengthwise, you could cover a regulation-sized tennis court with it.
And you needn't be some deranged butcher to appreciate this hole running through your body. Intact it is also quite impressive. When you drop some food into one end of the hole, it is exposed to a number of highly specialized materials that work to destroy it and reduce it to its components in a wide meadow called your stomach. Your external skin here in your stomach secretes a lot of mucous to protect it from being dissolved by the acid that digests food.
The stuff that food is reduced to (called chyme) gets passed along to your gut, which is the less technical term for your small and large intestines. Here, microbes go to work further digesting the nutrients and fiber in the chyme, fermenting it (and creating sulfuric gases as they do, which passes through one end or the other of your 27-foot hole). The skin on this outside region of your body is highly specialized and is capable of absorbing these nutrients and any water you digest. Once it passes through your gut walls, from outside your body to the inside, these nutrients are directed to various organs, all of which are located within your body.
All of the food stuff that wasn't digested is combined with some of the dead residents of the neighborhoods along this lengthy hole and is deposited into your rectum, a holding area for solid waste, where it will stay until it passes through the other end of the hole, your anus. All of this took place outside your body, which has evolved to include all of the curves, folds, juices and extraction capabilities that allow it to take full advantage of this food as it travels through the hole that runs through all of us.