When a person **is not breathing**, it is crucial to act quickly to help them. In this case, **artificial respiration** should be performed as part of cardiopulmonary resuscitation (CPR).

- **Cardiopulmonary Resuscitation (CPR):** It consists of performing chest compressions and mouth-to-mouth ventilations. This helps air enter the injured person's lungs and maintain blood circulation until emergency services arrive.

- **Procedure:**
1. Place interlocked hands on the injured person's chest.
2. Perform firm and rapid compressions, approximately 100 times per minute.
3. After every 30 compressions, perform 2 mouth-to-mouth ventilations.

- **Importance:** Artificial respiration is essential to maintain the flow of oxygen to the brain and other vital organs, which can be crucial for the injured person's survival.

Therefore, assisting the injured person with artificial respiration is the correct action when they are not breathing.