Chapter XI: WHY THE HEART BEATS.--Where is the heart? Why does

the heart beat? How many chambers has the heart? What are the blood-vessels? How many kinds of blood-vessels are there? Name them. What is the difference between venous blood and arterial blood? What change occurs in the blood in the lungs? What is the pulse? How much work does the heart do every twenty-four hours? What are the lymphatics? What do they contain, and what is their purpose? What are lymphatic glands?