Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called

Options

A. Fascicles
B. Myofibrils
C. Myofilaments
D. Both B and C

Explanation

To solve the question, we need to identify the term that describes the fine fibrils that compose the long cylindrical fibers of muscles. Here’s a step-by-step breakdown of the solution:

### Step-by-Step Solution:

1. **Understanding Muscle Structure**: Muscles are made up of long cylindrical fibers that are arranged in parallel arrays. These fibers are essential for muscle contraction and movement.

2. **Identifying the Components**: The question asks about the fine fibrils that make up these muscle fibers. We need to consider the options provided to find the correct term.

3. **Analyzing the Options**:
- **Option 1: Fascicles** - Fascicles are bundles of muscle fibers, not the fine fibrils that compose the fibers. Therefore, this option is incorrect.
- **Option 2: Myofibrils** - Myofibrils are the basic rod-like units of a muscle cell. They are indeed made up of long cylindrical fibers arranged in parallel arrays, making this option a strong candidate.
- **Option 3: Myofilaments** - Myofilaments are the smaller filaments that make up myofibrils. They are constructed from proteins and play a crucial role in muscle contraction. This option also seems relevant.
- **Option 4: Both B and C** - This option suggests that both myofibrils and myofilaments are correct answers.

4. **Conclusion**: Since myofibrils are the long cylindrical fibers that make up muscle cells, and myofilaments are the finer structures within myofibrils, both options 2 and 3 are correct in the context of the question. Therefore, the correct answer is **Option 4: Both B and C**.

### Final Answer:
The fine fibrils that compose the long cylindrical fibers of muscles are called **myofibrils** and **myofilaments**. Thus, the correct answer is **Option 4: Both B and C**.