In the rabbit, the expiration occurs by the

Options

A. Contraction of the external intercostal muscles and the relaxation of the muscles of the diaphragm
B. Contraction of the internal intercostal muscles and the relaxation of the muscles of the diaphragm
C. Contraction of the internal intercostal muscles only
D. Relaxation of the muscles of the diaphragm only

Explanation

To answer the question regarding how expiration occurs in rabbits, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Expiration**: Expiration, also known as exhalation, is the process of releasing air from the lungs. It is essential to understand the mechanics involved in this process.

2. **Role of Muscles**: During expiration, specific muscles are involved. The primary muscles that facilitate this process are the internal intercostal muscles and the diaphragm.

3. **Contraction of Internal Intercostal Muscles**: The internal intercostal muscles contract during expiration. This contraction pulls the rib cage downwards and inwards, reducing the volume of the thoracic cavity.

4. **Relaxation of the Diaphragm**: Simultaneously, the diaphragm relaxes. When the diaphragm relaxes, it moves upwards into a dome shape, further decreasing the volume of the thoracic cavity.

5. **Pressure Changes**: As the volume of the thoracic cavity decreases, the pressure inside the lungs increases compared to the atmospheric pressure. This pressure difference causes air to flow out of the lungs.

6. **Conclusion**: Therefore, in rabbits, expiration occurs primarily due to the contraction of the internal intercostal muscles and the relaxation of the diaphragm.

### Final Answer:
In the rabbit, expiration occurs by the contraction of the internal intercostal muscles and the relaxation of the muscles of the diaphragm.

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