Heart, the muscle that forces the blood to circulate. In man it is a cone shaped organ lying apex downward in a slanting position in the left side of the chest between the lungs. The point of the heart is opposite the space between the fifth and the sixth ribs. The base is opposite the second rib. It is a strong muscle with four cavities--two receiving and two expelling. The blood of the body drains into one side of the heart and is forced through arteries into the lungs. From the lungs it draws back into the other side of the heart and is squeezed out or driven to all parts of the body only to drain back as before. For an account of the auricles, ventricles, veins, and arteries, and especially of the valves which guard the doorways and prevent the backset of the blood, though permitting it to go forward, a physiology should be consulted. The heart is about the size of its owner's closed fist. Its action is due, it is held by high authority, to the influence of the blood upon the muscles. An accumulation of blood seems to exert an influence, electric or nervous, or whatever it may be, upon the muscular wall of the heart, causing a powerful spasmodic contraction which brings the walls of the cavity together and drives the blood out upon the next stage of its journey. The left side is the larger and more powerful, being charged with the circulation of the blood in the system. The contractions or beats of the heart are made at the rate of about seventy-two per minute. Some idea can be had of the work it must do when we recall that the blood weighs about one-twelfth as much as the entire body, and that it is pumped over and over, the entire amount passing through the heart in from fifteen to thirty seconds. It is hard to realize that while one slowly counts thirty, the blood has run swiftly from one's great toe to the heart, been pumped to the lungs, purified, drained back to the heart and driven to the great toe again, nor should it seem surprising that the action of the heart and the circulation of the blood were not understood by the most skillful surgeons until long after the discovery of America, not in fact till the more important American colonies were well underway. The heart of cold-blooded animals is comparatively small and feeble in action. The heart of the frog has two auricles and but one ventricle; that of the fish one auricle and one ventricle. See HARVEY; LYMPH; BLOOD; CIRCULATION.