Egestion

Whatever the small intestine fails to digest or absorb — mostly indigestible fibre, called roughage — passes on into the large intestine. There, symbiotic bacteria break down this roughage, and in the process manufacture useful substances such as vitamin K along with other metabolites, all of which the colon absorbs together with water reclaimed from the residue. What remains is progressively solidified into faecal matter as it collects in the rectum. Once enough faecal matter has accumulated, it stretches the rectal wall enough to trigger a neural reflex that produces the urge to defecate. The actual expulsion — defaecation — is, unlike most gut movements, under voluntary con …