This book is dedicated to one of the outstanding scientists of the 19th century, Ivane Tarkhnishvili (Tarchanoff, or Tarkhanov), a Russian physiologist of Georgian origin who graduated from the St. Petersburg Medical-Surgical Academy (1869) and worked under the guidance of the founder of Russian physiology, Ivan Sechenov. Among his many achievements was the discovery of the galvanic skin reflex (1889). However, Tarkhnishvili's equally significant contribution to biomedical science was the discovery of the effect of X-rays on embryonic development, heart and blood circulation, the central nervous system, and animal behavior. Indeed, these works laid the foundation for a new field of science - radiobiology. Studying the higher functions of the brain, Tarkhnishvili tried to bridge the gap between physiology and psychology, which is fully described in his last book "Spirit and Body" (1904).