Final answer:
The medulla is the brain structure that was likely damaged in the accident, resulting in Chris's inability to send messages for breathing.
Step-by-step explanation:
The brain structure that was likely damaged in the accident is the medulla. The medulla, along with the pons, makes up the hindbrain, or brainstem. It is responsible for regulating several vital functions, including breathing, blood pressure, heart rate, and digestion. In this case, the damage to the medulla has affected Chris's ability to send messages for him to breathe, leading to the need for a respirator.