Which respiratory muscles are called principal respiratory muscles and why?

Explanation

**Step-by-Step Solution:**

1. **Identify the Principal Respiratory Muscles:**
The principal respiratory muscles are primarily the diaphragm and the external intercostal muscles.

2. **Function of the Diaphragm:**
The diaphragm is a dome-shaped muscle located at the base of the thoracic cavity. When it contracts, it flattens and moves downward, increasing the volume of the thoracic cavity and allowing air to be drawn into the lungs.

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