In vertebrates the myocardium refers to

Options

A. Skeletal muscles
B. Muscular wall of heart
C. Cells of pericardium
D. Fluid of pericardium

Answer

### Step-by-Step Solution:

1. **Understanding Myocardium**: The term "myocardium" refers to the heart muscle in vertebrates. It is also known as cardiac muscle.

2. **Types of Muscle in Vertebrates**: In vertebrates, there are three types of muscle: skeletal muscle, smooth muscle, and cardiac muscle (myocardium). The myocardium is distinct from the other two types.

3. **Function of Myocardium**: The myocardium is responsible for involuntary functions, meaning it operates without conscious control. This is crucial for the heart's ability to pump blood continuously.

4. **Structure of Myocardium**: Myocardium is characterized by striated muscle fibers, which is why it is referred to as striated muscle.

5. **Location of Myocardium**: The myocardium forms the thick middle layer of the heart wall. It is situated between two other layers: the outer layer known as the epicardium and the inner layer known as the endocardium.

6. **Composition of Myocardium**: The myocardium is made up of individual heart muscle cells called cardiomyocytes. These cells are interconnected by structures known as intercalated discs, which facilitate synchronized contractions.

7. **Surrounding Structures**: The cardiomyocytes are surrounded by collagen fibers and other substances that form the matrix of the myocardium.

8. **Identifying the Correct Answer**: Based on the information provided, the myocardium is best described as the muscular wall of the heart. Therefore, the correct answer to the question is option 2: Muscular wall of the heart.