Sitting on a toilet is a 19th-century novelty; most people squat
The Western sit-down toilet feels universal, but it only spread in the 19th century with indoor plumbing. Most of the world, including much of Africa, Asia and the Middle East, uses squat toilets. Behind that everyday choice lies a surprisingly intricate reflex, part automatic and part voluntary, that the body takes years to bring under control.
Waste gathers in a chamber at the end of the rectum called the ampulla. As it fills, stretch sensors in the rectal wall create the urge to go, triggering the rectal muscles to contract and the inner anal sphincter to relax, while the outer sphincter briefly tightens. If the moment is inconvenient, the material is often pushed back into the colon by reverse muscle waves, where more water is drawn out, until the next big sweep of movement arrives.
The two sphincters obey different masters. The internal one works involuntarily, while the external one is under conscious control. Infants empty their bowels purely by reflex, and toilet training is the process of learning to govern the outer muscle. That control can later be lost through nerve or physical injury, surgery, disease, intense fright, childbirth or death.
Human frequency varies widely, from several times a day to a few times a week, depending on habits, environment and genes. Waves of contraction called peristalsis move material through the colon. Delay too long and stools harden into constipation; move things too fast, before water is absorbed, and the result is diarrhoea. Researchers have linked chronic constipation with higher risk of heart disease and stroke. Babies pass a unique first substance, meconium, before they begin eating, and birds expel urine and faeces together in one mass.
The subject is awkward enough to have spawned a huge vocabulary, from the clinical bowel movement to euphemisms like 'number two', and it fuels a whole genre of scatological humour. It also matters for public health. Faeces can spread disease, often by contaminating food, with E. coli a particular worry. Open defecation remains widespread in some developing countries, while wealthier ones rely on sewage treatment.
Source: Defecation
