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Foetus heart. Coloured image of a section through the chest of a foetus showing its healthy heart (upper centre left). The front of the chest of the foetus is at top. On either side of the heart are the lungs, with the trachea (windpipe) and blood vessels resembling hollow pipes at lower centre. The chest is surrounded by the bones of the ribcage, which meet at the vertebra at bottom centre. The heart has four chambers, two large ventricles (upper left) and two atria (seen in section). (KEYSTONE/SCIENCE PHOTO LIBRARY/DR G. MOSCOSO)