I'm siding with mcfreddy. A muscle...here's why: I was given a prescription for a muscle relaxer and was told by the doctor to not take any extra doses (even if the first dose didn't work) because the heart is a muscle and too much muscle relaxer would be dangerous or even fatal.
The heart is an organ and this is the reason why. An organ is composed of tissues with a specific function. The heart contains muscle tissue in its myocardium, fibrous tissue in its pericardium, nervous tissue in its innervation, blood and blood vessels such as the coronary arteries which supply blood to the heart. These different types of tissues all help the heart perform one specific function, that is, to effectively pump blood from the venous circulation to the lungs and then to the arterial circulation.