Name the body structure concerned with the given functional activity :
Helps to increase the volume of the chest cavity lengthwise.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the body structure that helps to increase the volume of the chest cavity lengthwise.

2. **Identifying the Relevant Body Structure**: The diaphragm is a key muscle involved in respiration. It separates the thoracic cavity (where the lungs are located) from the abdominal cavity.

3. **Function of the Diaphragm**: When we inhale (inspiration), the diaphragm contracts and moves downward. This contraction increases the volume of the thoracic cavity, allowing the lungs to expand and fill with air.

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