Life, either as we think of it in the abstract in its highest sense, or life, as we think of it in terms of a compact living organism, is obviously the result of complex interaction of all of the components of the organism. One could therefore question the advisability of separating out the nervous system for a special detailed study in our age of overspecialization. The main purpose of the present Handbook is not to fragment further our approach or under- standing of living phenomena, but, on the contrary, to try to summarize and integrate as much of the available information and thinking on the nervous system as is possible in a limited space. It is difficult to think of an area of modern biology that is more exciting to study and that has greater impor- tance for mankind, from any point of view, than the study of the brain and of the nervous system. The influence that understanding of brain function in biological terms can exert on our future is not generally understood in its full impact. Although our ignorance about even the most basic mechanisms in the nervous system is enormous, in recent years our knowledge has made most important advances, and as a consequence great masses of data have been accumulated.