The heart is a hollow muscular organ that by its rhythmic contraction acts as a force pump maintaining the circulation of the blood. It is enclosed in a serous pericardium ,and consists of four chambers divided into an upper pair of rather thin walled auricles which - walled ventricles into which the blood is forced and which in turn pump it into the arteries , back flow being prevented by valves. The abdomen contains the stomach , pancreas , liver ,