When I was three or four years old, I read a series of picture books about anatomy which my mother and I found at the library. This was way back when my mother's sole purpose in life was brainwashing me to become a doctor. There was a book for every system in the body. My favorite was the digestive system because it wasn't uncomfortably rude like the reproductive system, but it was still rude.
It's strange how some things stick in your head. The book guided the intrepid reader through the mouth and the esophagus, down into the stomach and duodenum, through the meters of small intestine, the absurd-looking large intestine, and finally into the rectum. And one sentence in the book, after twenty-two years, still sticks in my mind.
"Life would be very difficult without the anal sphincter."
I was so confused by this sentence that I asked my mother why life would be difficult without the anal sphincter. She hypothesized that without a circular squeezing bottom muscle, we would have to cut our poo into chunks with a pair of scissors as it came out. Yes, I thought, life surely would be difficult if we had to manually cut our poo into chunks with a pair of scissors.
These things go through your head when you're three years old, and also when you haven't slept enough.
Of course, she's wrong. The real reason life would be difficult is that poo would leak out of your rectum all the time since there would be nothing sealing it in. Perhaps we'd all have to wear corks or something. I guess she didn't think of that.