Final answer:
The middle layer of the heart is known as the myocardium, which is comprised of muscle cells and is crucial for the heart's pumping action. The endocardium is the inner lining, and the epicardium is the outer layer that forms part of the protective pericardium.
Step-by-step explanation:
The middle layer of the heart is called the myocardium. The myocardium consists of heart muscle cells that make up the middle layer and the bulk of the heart wall. This is the layer responsible for the contraction and beating of the heart, consisting mainly of cardiac muscle tissue. The myocardium is pivotal for the heart's ability to pump blood through the circulatory system. It is supplied with blood vessels, and nerve fibers, and also contains pacemaker cells that are essential in regulating heart contractions.
The other layers of the heart include the endocardium, which is the inner lining of the heart and the epicardium, the outer layer that is also a part of the pericardium. The pericardium is the membranous layer that surrounds and protects the heart, and it is made up of two membranes with a fluid-filled cavity in between to reduce friction and allow efficient heart movements.
The middle layer of the heart is called the myocardium. It consists mainly of cardiac muscle tissue and makes up the bulk of the heart's wall. The myocardium is responsible for the beating of the heart, as it contains muscle cells that contract to pump the blood.