In this paper, we study a cell of the subdivision induced by a union of n half lines (or rays) in the plane. We present two results. The first one is a novel proof of the O(n) bound on the number of edges of the boundary of such a cell, which is essentially of methodological interest. The second is an algorithm for constructing the boundary of any cell, which runs in optimal Θ(n log n) time. A byproduct of our results are the notions of skeleton and of skeletal order, which may be of interest in their own right.