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anatomy system digestive human muscles education skeletal appendicular skeleton terminology bones appendicular distance liver intestines stomach digestion articulations integumentary skin histology embryology anatomical body regions cells organelles human evolution science history skull skeleton creativity tina_seelig stanford respiratory joints introduction body regions heart veins arteries autonomic nervous system anatomy nervous system autonomic ans anatomy nervous system pns anatomy nervous system cns axial skeleton ribs vertebrae arm shoulder humerus ulna radius tibia femur fibula pelvis cartilage development evolution