Which of the following events is not related to inspiration?

Options

A. Increase in the volume of the thoracic cavity
B. Relaxation of external intercostal muscles
C. Contraction of external intercostal muscles
D. Contraction of the diaphragm

Answer

To solve the question "Which of the following events is not related to inspiration?", we need to analyze the given options and understand the process of inspiration (inhalation) in the respiratory system.

### Step-by-Step Solution:

1. **Understanding Inspiration**:
- Inspiration is the process of taking air into the lungs. It involves several physiological changes that facilitate the intake of air.

2. **Identifying the Events**:
- The options provided are:
- A) Increase in the volume of the thoracic cavity
- B) Relaxation of external intercostal muscles
- C) Contraction of external intercostal muscles
- D) Contraction of the diaphragm

3. **Analyzing Each Option**:
- **Option A: Increase in the volume of the thoracic cavity**:
- This is related to inspiration. During inhalation, the diaphragm contracts and moves downwards, and the external intercostal muscles contract, causing the rib cage to expand. This results in an increase in the volume of the thoracic cavity.

- **Option B: Relaxation of external intercostal muscles**:
- This is not related to inspiration. In fact, during inspiration, the external intercostal muscles contract to help expand the thoracic cavity. Their relaxation occurs during expiration (exhalation).

- **Option C: Contraction of external intercostal muscles**:
- This is related to inspiration. The contraction of these muscles helps lift the rib cage and expand the thoracic cavity, allowing air to be drawn into the lungs.

- **Option D: Contraction of the diaphragm**:
- This is also related to inspiration. The diaphragm contracts and moves downwards, increasing the thoracic cavity's volume and reducing pressure, allowing air to flow in.

4. **Conclusion**:
- Among the options, the event that is **not related to inspiration** is **B) Relaxation of external intercostal muscles**. This event is associated with expiration.

### Final Answer:
**B) Relaxation of external intercostal muscles**