What is the majority of the mass of the heart composed of?

a. Endocardium
b. Myocardium
c. Pericardium
d. Endothelium

Since the heart is a big muscle and "myo-" refers to muscle, guess which one I would choose.

myocardium

The majority of the mass of the heart is composed of:

b. Myocardium

To determine the majority of the mass of the heart, one needs to understand the different layers that make up the heart.

The heart is composed of three layers: the endocardium, myocardium, and pericardium.

The endocardium is the innermost layer of the heart. It is made up of a thin layer of endothelial cells, connective tissue, and smooth muscle. However, the endocardium does not contribute significantly to the overall mass of the heart.

The pericardium is the outermost layer of the heart. It is a protective sac that surrounds the heart and helps to hold it in place within the chest cavity. The pericardium is composed of two layers: the fibrous pericardium and the serous pericardium. Like the endocardium, the pericardium does not contribute significantly to the overall mass of the heart.

The myocardium is the middle layer of the heart and is responsible for the heart's contractions. It is primarily composed of specialized cardiac muscle cells called cardiomyocytes. The myocardium is the thickest layer and makes up the majority of the mass of the heart.

Therefore, the correct answer is b. Myocardium.