Chemical and mechanical theories of digestion in early modern medicine.

The aim of this paper is to survey the iatrochemists' and iatromechanists' explanations of digestion, from the sixteenth to the early decades of the eighteenth century. The iatrochemists substituted the Galenic thermal digestion with a series of chemical processes, the same as those produced in the laboratory. Jean Baptiste van Helmont marked a turning point in the chemical understanding of digestion, indicating the acid ferment in the stomach as the digestive agent. In the wake of van Helmont, an increasing number of physicians rejected the traditional Galenic theory of digestion, turning to the chemical reactions taking place in the ventricles. The iatrochemists saw nutrition as the outcome of the separation of an active invisible substance, i.e., spirits, from a thick inert covering. The emergence of the mechanical physiology, with its emphasis on the shape, size and motion of parts, did not bring about a decline of the chemical investigations of digestion. Descartes ruled out chemistry in the study of physiology, while a number of physiologists-notably in England-adopted a compromise between iatrochemical and mechanical theories. In the second half of the seventeenth century, the view of acid as an agent of gastric digestion became popular among physiologists. Late in the century, the acid-alkali doctrine spurred further investigations on digestion and nutrition.