The weight of shit stretching the sides of the sigmoid colon signals the top sphincter muscle to relax briefly allowing a measured amount into the rectum. This is also called a movement. If you’re ready to go and sitting on a toilet, the bottom sphincter muscle relaxes and lets the shit down the anal canal and out through your arsehole.
Defecation | Wikipedia
Bowel movement: the push to change the way you poo | The Guardian 30 Nov 2018
↑ Back to top