Each repetition of its back and forth motion is a little less vigorous than its previous repetition. If the head sways 3 cm to the right of its equilibrium position during the first repetition, it may only sway 2.5 cm to the right of its equilibrium position during the second repetition.
The atria and the ventricles are separated by valves, which allow the blood to flow through the heart in one direction and prevent back flow of blood. The wall of the heart is made up of three layers. The outer layer is called the epicardium. The middle layer is the actual heart muscle and is called the myocardium. The inner layer of the heart is called the endocardium. The heart is contained within a sac called the pericardium. The sole purpose of the heart is to pump blood to and from the rest of the body.
The heart wall consists of three layers: 1  Epicardium - the outer layer of the wall of the heart. 2  Myocardium - the muscular middle layer of the wall of the heart. 3  Endocardium - the inner layer of the heart.