Many parts of the brain evolved after the "reptilian". And to keep it simple, I will just say that the "mind" is the result of the functioning of the brain. I believe that we are spiritual beings, therefore I am of the "Dualist" school of thought, i.e., the mind is the interaction of brain and spirit.
I think of the mind as the "drama queen" of friends. You know the one. She often overreacts to information, dramatizes, and disses. She never lets go of a topic, even if it is hurtful. She talks, talks, talks, long after you wish she would just be quiet. (Especially when you are trying to go to sleep.) She can be repetitive and scary. She has some really terrifying stories to tell of what could happen to you. On the other hand, when she is feeling good about herself, she is a best friend. If you can let her know when she is out of line, she calms down, and does what you need her to do. But she's not easy to control.
So how does that relate to the "reptilian overlord"?
When our senses detect something as dangerous, such as the loss of our retirement, or a desirable woman your husband notices, or that scary man in the alley, it sends signals to your brain that interacts with your friend, getting her all worked up. As she goes into full-on drama mode your body responds with appropriate hormonal activity. Now here is the catch, the brain doesn't know the difference between something made up, (imagined), and something that is actually happening. But that's another story. Suffice to say, when the more basic part of our brain is activated, we can respond in ways that our more reasonable parts are quite shocked at, mainly because our "dramatic" friend is egging us on. The trick is to know when she is right and you better listen, when she needs to calm down and be quiet, and when to run.