NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are draw from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of member of the National Academy of Sciences, the National Academy of Engineeing, and the Institute of Medicine.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate profession in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Science by its congressional charter to be an adviser to the federal government and, upon its own initiative to identify issues of medical care, research, and education Dr. Kenneth I.Shine is president of the Institute of Medicine.

Support of this activity was provided by the Howard Hughes Medical Institute. The opinions and conclusions expressed here are those of the author and do not necessarily represent the views of the Howard Hughes Medical Institute, the National Academy of Sciences, or any of their constituent parts.

Additional copies of this report are available in limited quantities from the Board on Health Sciences Policy, 2101 Constitution Avenue, N.W., Washington, DC 20418.

The serpent has been a symbol of long life, healing, and knowledge among almost all cultures and religions since the beginning of recorded history. The image adopted as a logotype by the Institute of Medicine is based on a relief carving from ancient Greece, now held by the Staatlichemuseen in Berlin.