HARDto believe this one ….
Muscle move or propel the waste, there are stretch-detecting nerve endings in the rectum. These nerve detect when this area is full ,the initial sphincter opens briefly and allows a tiny bit of rectum's contents come into contact with the external sphincter, which is rich with nerve endings. In a rapid "SAMPLING REFLEX" the nerves inform the brain about whether the rectum contents are intestinal gasses, or liquid or a solid stool.
This allows you to act accordingly--------------------- You might allow gases to escape,, or be looking for a restroom , right away if happen to have a case of diarrhea or waite for a more convenitent time and place.