How to extract specific region from black and white image using openCV

I have been trying to learn OpenCV as I have a deep interest in Computer Vision and one of the problems I have been trying to figure out is how to extract a particular region of an image with OpenCV. I do not want to merely crop the image; I want to extract the exact region from it. For example if I have an image of the lower half of the face like this:

Is there a way I can extract the just the lower lip from this image. Any insight will be much appreciated, thank you!