ogy, of Schwann and Henle in anatomy, of Du Bois-Reymond and Helmholtz in physiology and of Virchow in pathological anatomy. It is not surprising therefore that it was the proud boast of this school that not only had it dispelled the vague notions of the old metaphysical school and established in its stead the true scientific spirit, but that it had filled so many of the chairs of medicine, physiology and anatomy in the German universities that the scientific spirit has been applied to "every branch of medical science, which it has in consequence drawn into the circle of the exact or mechanical sciences." (Merz.)
This is not the place to go into detail concerning the investigations of Müller and his school of physiology. His law of "specific energies," Du Bois Reymond's electro-physiology and Helmholtz's work on musical acoustics and physiological optics indicate the character and scope of the work. The keynote of it all Müller himself has expressed in his "Elements of Physiology" as follows:
Though there appears to be something in the phenomena of living beings which can not be explained by ordinary mechanical, physical or chemical laws, much may be explained, and we may without fear push these explanations as far as we can, so long as we keep to the solid ground of observation and experiment.
These principles and the labors of this school were advanced wonderfully, in 1847, by Ludwig's invention of the kymograph and the elaboration of methods of graphic registration, factors which established this phase of physiology on a sound basis and exerted an influence which medicine feels to this day. This, however, was not the only influence of Müller. As a biologist with general interests he stimulated general biological research and it was undoubtedly this influence exerted through Schwann that led the latter to grasp the importance of Schleiden's work on vegetable cells and to apply the observations of the latter to the cells of the animal body.
But although the cell doctrine, in its modern conception, is the result of the work of these two men, Schleiden and Schwann, it is not to be supposed that they were the first to study cells, for before Schleiden considerable attention had been given to the structure of vegetable tissues. Robert Hooke in 1665 had given to the spaces in cork and similar structure the names of "cells"; Malpighi (1674) and Grew (1683) had, as far as their low power lenses would allow, described plant tissue as made up in part of cell-like cavities provided with firm walls and filled with fluid, and in part of long tube-like vessels. Treviranus, in 1806, demonstrated that these tubes arose as the result of cells becoming attached end to end, the intervening ends eventually disappearing. The nucleus of the cell had been discovered in 1831 by Brown, who, however, failed to realize its importance. Not so Schleiden. He attached great importance to the nucleus and by the numerous observa-
