The physiologist, the zoölogist, the botanist, the psychologist, the pathologist, have to deal, day in, day out, with the effects of stimulation on the living substance. No living substance exists without stimulation. In the vital manifestations of all organisms the interplay of the most varied stimuli produces an enormous and manifold variety of effects. Experimental biological science employs artificial stimulation as the most important aid in the methodic production of certain effects of stimulation. The number of researches in which special effects of stimulation are treated is endless. Nevertheless the systematic investigation of the effects of stimulation have, curiously enough, been strangely neglected. Although countless results of individual effects of stimulation have been studied, the attempt has never been made to establish a general physiology of the laws of stimulation and consider it as an independent problem. This circumstance induced me to systematically investigate the general laws of the effect of stimulation. In the fifth and sixth chapters of my book on general physiology the results of these studies are recorded for the first time. Since then, especially during our own researches on the general physiology of the nervous system, a great number of new facts of importance for the general physiology of the effects of stimulation have been obtained. All these results I have endeavored to combine and elucidate in the following lectures. The text of the lectures in its present form was written in German in 1911. The English translation was made by my wife, with the help of our friend, Dr. Lodholz of the University of Pennsylvania, who also undertook the reading of the proofs. We wish here to thank him once again and express our deep appreciation of the great sacrifice of time and labor involved in this task. I am likewise much indebted to Dr. Julius Vészi for his assistance unstintingly given, especially in obtaining a number of curves. Finally, I wish to take this opportunity to render warmest thanks to the authorities of Yale University, and especially to President Hadley and Professor Chittenden, as well as to my special colleagues, for the hospitality and cordial reception extended to me in New Haven and for the pleasant hours I was privileged to spend in their midst. MAX VERWORN. Bonn. Physiological Laboratory of the University. CONTENTS I