Acid alone does not digest: a lesson from nineteenth-century stomach physiology

"Gastric acid" and "gastric juice" are commonly used as interchangeable terms, thus obscuring a distinction that took physiologists generations to establish. Hydrochloric acid denatures dietary protein, activates pepsinogen, and maintains the pH at which pepsin is active - but it does not itself hydrolyze peptide bonds (i.e., digest). Disentangling the interplay between acid and pepsin required independent, near-simultaneous work in Germany and the United States in the early 1830s, followed by Theodor Schwann's isolation, in 1836, of a separate heat-sensitive agent he named pepsin. The elimination logic Schwann used to reach that conclusion - testing and ruling out each plausible alternative in turn - offers a reasoning model still worth teaching today.