How was mechanical digestion discovered?

Borelli made birds swallow objects that he then found crushed, deformed and broken after digestion. This showed that digestion involves mechanical transformations.

Borelli compared the action of jaws to pincers and that of the stomach to a vice.

How was chemical digestion discovered?

Réaumur placed a piece of meat inside an iron pipe and made a buzzard swallow it. After digestion, the pipe was intact but the meat was almost completely broken down. This proved that digestion happened without grinding and was the first step towards understanding chemical digestion.