ening up the paths that
were to lead to the wonderful cell theory, another novel line of
interrogation of the living organism was being put forward by a
different set of observers. Two great schools of physiological chemistry
had arisen--one under guidance of Liebig and Wohler, in Germany, the
other dominated by the great French master Jean Baptiste Dumas. Liebig
had at one time contemplated the study of medicine, and Dumas had
achieved distinction in connection with Prevost, at Geneva, in the
field of pure physiology before he turned his attention especially to
chemistry. Both these masters, therefore, and Wohler as well, found
absorbing interest in those phases of chemistry that have to do with the
functions of living tissues; and it was largely through their efforts
and the labors of their followers that the prevalent idea that vital
processes are dominated by unique laws was discarded and physiology was
brought within the recognized province of the chemist. So at about
the time when the microscope had taught that the cell is the really
essential structure of the living organism, the chemists had come to
understand that every function of the organism is really the expression
of a chemical change--that each cell is, in short, a miniature chemical
laboratory. And it was this combined point of view of anatomist and
chemist, this union of hitherto dissociated forces, that made possible
the inroads into the unexplored fields of physiology that were effected
towards the middle of the nineteenth century.

One of the first subjects reinvestigated and brought to proximal
solution was the long-mooted question of the digestion of foods.
Spallanzani and Hunter had shown in the previous century that digestion
is in some sort a solution of foods; but little advance was made upon
their work until 1824, when Prout detected the presence of hydrochloric
acid in the gastric juice. A decade later Sprott and Boyd detected
the existence of peculiar glands in the gastric mucous membrane; and
Cagniard la Tour and Schwann independently discovered that the really
active principle of the gastric juice is a substance which was named
pepsin, and which was shown by Schwann to be active in the presence of
hydrochloric acid.

Almost coincidently, in 1836, it was discovered by Purkinje
and Pappenheim that another organ than the stomach--namely, the
pancreas--has a share in digestion, and in the course of the ensuing
decade it came to be known, thr