The video content presented here requires JavaScript to be enabled and the latest version of the Macromedia Flash Player. If you are you using a browser with JavaScript disabled please enable it now. Otherwise, please update your version of the free Flash Player by downloading here.


If the table of contents links listed above do not function in your browser, please use the slider in the Flash player to advance through the presentation.
 
Should you have any difficulties with the presentation, please contact sackler@nas.edu.
 
Copyright © 2006. National Academy of Sciences. All rights reserved.