homeomorphism

Let $B$ be the intersection of the sphere $S^2$ with the first octant $\{(x_1,x_2,x_3)|x_1\geq0,x_2\geq0,x_3\geq0\}$ of $\mathbb{R}^3$ . The book says $B$ is homeomorphic to the ball $B^2$ . But I'm not sure why that is, can anyone explain this please. Thank you very much.