The reticular activating system (or RAS) is the part of the humanoid brain that regulates respiration, cardiac function, body temperature, and other fucntions. When Fabian Stevens was infected by an Ardanan parasite in 2377, Dr. Elizabeth Lense was concerned that it could extend to and inhibit the functions of Stevens's RAS, resulting in death. The parasite eventually did reach this part of his brain; Lense and Dr. Sarjenka were able to kill it of, though, by inducing a coma in Stevens. (CoE eBook: Signs from Heaven)