The heart is a muscular organ, in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδία, kardia, for "heart." The heart of a vertebrate is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. The aver… (More on Cardiac)