PHYSIOLOGICAL PSYCHOLOGY--J. MÜLLER, HELMHOLTZ, WEBER, FECHNER
TURNING now to our second heading of the anatomy and physiology of the nervous system, we find that, in the period 1833-1860, progress lay chiefly in the direction of a better understanding of the detailed structure and function of individual nervous units rather than in fresh discoveries concerning localization of function in the brain. We have seen how, just at the beginning of this period, improvements in the microscope led to a clear differentiation between cells and fibres and a realization of the fact that the grey matter of the brain was composed largely of cells. Histological work of this kind was carried on vigorously throughout the period, stimulated from time to time by the discovery of fresh methods of making microscopical preparations. In this way the details of cell structure gradually became clear. Meanwhile, physiological research was throwing light upon the functional relations of the different parts of a nerve. In 1839 Nasse found that if a nerve trunk is severed in the middle of its course, the peripheral end degenerates, while the central end does not. Thirteen years later, in 1852, Waller interpreted this fact to mean that every fibre was connected with a nerve cell and that the cell has some sort of trophic function. Waller also showed that the so-called "secondary degeneration" of the peripheral end of the severed nerve could be used as a very valuable clue whereby to trace the course of the nerve, by following the line of degeneration, wherever it might lead. In this way it became possible to plot the course of nerve, with an ease and accuracy hitherto impossible.