Let $D \subset \mathbb{R}^n$ be a bounded domain. If $\partial D$ is a realanalytic submanifold, it is not very difficult to show that $\partial D$ does not contain any line segments. Now, assume that $\partial D$ is a finite disjoint union of (connected) realanalytic submanifolds of $\mathbb{R}^n$ such that there is precisely one submanifold of dimension $n1$, say $B$. Is it true that $B$ does not contain any line segments?
