Proctologist W.C. Bornemeier explains why the sphincter ani must be preserved when performing hemorrhoid surgery:

They say man has succeeded where the animals fail because of the clever use of his hands, yet when compared to the hands, the sphincter ani is far superior. If you place into your cupped hands a mixture of fluid, solid and gas and then through an opening at the bottom, try to let only the gas escape, you will fail. Yet the sphincter ani can do it. The sphincter apparently can differentiate between solid, fluid and gas. It apparently can tell whether its owner is alone or with someone, whether standing up or sitting down, whether its owner has his pants on or off. No other muscle in the body is such a protector of the dignity of man, yet so ready to come to his relief. A muscle like this is worth protecting.

— W.C. Bornemeier, “Sphincter Protecting Hemorrhoidectomy,” American Journal of Proctology 11 (1960), 48-52