Robert B. Trelease, PhD, Professor, Division of Integrative Anatomy, department of Pathology and Laboratory Medicine, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California ; illustrations by Frank H. Netter, MD ; contributing Illustrators, Carlos A.G. Machado, MD, Kristen Wienandt Marzejon, MS, MFA, Tiffany DaVanzo, MA, CMI, John A. Craig, MD