This fourteenth-century hand-colored woodcut from Germany depicts a wheel that classifies urine samples according to their color and consistency. In the middle of the wheel, a doctor inspects a patient’s urine by sight, smell, and taste. The vials on the wheel aided physicians in diagnosing diseases. A key component of urine is urea, which is formed from the amino groups released during the metabolism of amino acids.

[(Left) Rosenwald Collection. Rare Book and Special Collections Division, Library of Congress (128.2).]