The directed distance $d(u,v)$ from $u$ to $v$ in a strong digraph $D$ is the length of a shortest $u-v$ path in $D$. The eccentricity $e(v)$ of a vertex $v$ in $D$ is the directed distance from $v$ to a vertex furthest from $v$ in $D$. The center and periphery of a strong digraph are two well known subdigraphs induced by those vertices of minimum and maximum eccentricities, respectively. We introduce the interior and annulus of a digraph which are two induced subdigraphs involving the remaining vertices. Several results concerning the interior and annulus of a digraph are presented.
 G. Chartrand, G. L. Johns, S. Tian and S. J. Winters: The interior and the annulus of a graph
. Congr. Numer. 102 (1994), 57–62. MR 1382357