The structure which does not contribute to breathing movements in mammals is :

Options

A. ribs
B. diaphragm
C. larynx
D. abdominal muscles

Explanation

To determine which structure does not contribute to breathing movements in mammals, we need to analyze the roles of various structures involved in the process of breathing.

### Step-by-Step Solution:

1. **Understanding Breathing Mechanism**: Breathing or ventilation is essential for the respiratory system to facilitate gas exchange. It involves moving air to the lungs where gas exchange occurs in the alveoli.

2. **Key Structures in Breathing**:
- **Diaphragm**: This is a large dome-shaped muscle located below the lungs. It plays a crucial role in breathing. When the diaphragm contracts, it increases the volume of the thoracic cavity, allowing air to be drawn into the lungs (inspiration). When it relaxes, air is expelled from the lungs (expiration).
- **Intercostal Muscles**: These muscles are located between the ribs and assist in expanding and contracting the rib cage during breathing. They contribute to both inspiration and expiration, especially during forceful breathing.
- **Abdominal Muscles**: These muscles also play a role in forceful expiration by pushing the diaphragm upwards, thus expelling air from the lungs more forcefully.

3. **Identifying Non-Contributing Structure**:
- **Larynx**: The larynx, also known as the voice box, does not have any muscular structures that contribute to the mechanical process of breathing. Its primary function is sound production due to the presence of vocal cords. It does not assist in the contraction or relaxation needed for ventilation.

4. **Conclusion**: Among the structures mentioned, the larynx does not contribute to breathing movements in mammals.

### Final Answer:
The structure which does not contribute to breathing movements in mammals is the **larynx**.

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