It has long been known that prolonged activity of mucous gland cells results in characteristic histologic changes in these cells, owing to the disappearance from the cell of certain granules (zymogen granules). This granular material is evidently used to make organic material of the secretion. It was doubtless on the basis of such observations that certain physiologists were led to seek for similar alterations in the highly specialized nerve cell following functional activity. Many of the investigations of these physiologists were confined to one or two experiments, the material was often not sufficiently controlled by normal tissue for comparison, and frequently the histologic technic was faulty.
There is the utmost divergence of opinion as to the nature of the changes taking place in nerve cells following activity, and one who tries to correlate the findings of the different workers in this field is utterly confused. The chief findings relate to