Sorry if this is really obvious. I've tried looking at pictures, but can't quite work it out.
When sat down, in the back of my leg I can feel two big chunky non-bone things which seem to attach to my knees at one end, and disappear into the back of my upper leg (arse end). There's two per knee, with the outside one being bigger. Hope I didn't use too much jargon there. What are these called, please?