The first edition of this text-book of physiology appeared in 1it.')!). It was reprinted in the difficult war years, while in 19.11-53 much of it was rewritten. For the present edition some parts have been abridged and a number of changes and additions made.In preparing this text-book, the authors sought to reflect as fully as possible all the fundamentally new ideas introduced into physiology by I. P. Pavlov, our teacher and renowned naturalist. Pavlov paved the way for an understanding of the physiological processes in an integral, normal organism that is in natural contact with Ihc external environment. This was achieved by Pavlov not only by introducing and systematically applying his absolutely novel methods of research. Of still greater importance in the creation of a new physiology—the physiology of an integral organism—was Pavlov's new principle of investigation: this required that all phenomena in an organism be studied in their dependence on the nervous system which through reflexes establishes all the connections of the animal organism with surrounding nature. Precisely this principle underlies Pavlov's great teaching on conditioned reflexes, which extended ihe principle of scientific determinism to a comprehension of the most intricate phenom­ ena in the world—a knowledge of all forms of activity of the higher parts of the brain, including consciousness.From this it follows (and we have endeavoured to show il) that the integrating activity of the nervous system, in the true sense of the word, can be understood only if we examine the nervous activity in its entirety as based on the reilex mechanism, in the new, Pavlovian concept. Normally each retlex act is an indissoluble union of conditioned .and unconditioned reflexes; these reflex acts determine the character and course of all physiological processes which depend on the concrete conditions of the existence of animals and of man's activity.Such are the fundamental principles which we have tried, within the limits of available factual material, to reflect in this book. The authors realize that; so far they have not fully coped with their task. However, their attempt is justified if only by the fact that even an outline oT the significance of the principles underlying Pavlov's teaching may in a certain measure help to disseminate them.