The modern era of research in cardiac muscle began in the late fifties, when Abbott and Mommaerts (J. Gen. Physiol. 42: 533, 1959) began experiments with isolated strips of cardiac muscle. Unlike the situation with skeletal muscles, it is not possible to isolate strips of tissue from the heart with tendons at either end, the kind of specimen one prefers for mechanical experiments. The most useful preparation has been the papillary muscle, which interconnects the ventricular endocardial wall with the atrioventricular valves. This tissue is easily isolated, has one tendon, and at least in rare specimens may have a relatively uniform cylindrical shape.