“Essentially, all models are wrong, but some are useful.” —George Box

Our “reptilian brain” is the deepest and most fundamental part of the brain used by all animals from reptiles on up the evolutionary ladder. It handles our automatic reactions—breathing, hormone and blood regulation, the startle response are all initiated from the reptilian brain. There is no emotion here, only instinct, and it is out of our control. Reptiles and birds thrive with this brain.

Source: Were yoga’s ancient sages insane?