Final answer:
The thick, fibrous inner layer of the heart is known as the myocardium, and it plays a crucial role in the heart's pumping function.
Step-by-step explanation:
The thick, fibrous inner layer of the heart is called the myocardium. This is the middle layer of the heart and its main component is cardiac muscle. The myocardium is vital for the contractile function of the heart, as it contains the heart muscle cells responsible for the pumping action. Surrounding the myocardium is the epicardium, which is the outermost layer and a part of the pericardium that helps to protect the heart. Inside the myocardium is the endocardium, which is the innermost layer lining the heart's chambers and valves, composed of simple squamous epithelium known as endothelium.