A vascular system consists of a supplying arterial and a draining venous part which are connected by a terminal vascular network. The arterial segment can be characterized according to the structural features of the vessel wall. However, it is sometimes diffi- cult to distinguish the capillary from the postcapillary vessels on the basis of structural features alone. On the other hand, physiologic qualities such as permeability can hard- ly be associated with an equivalent histologic pattern of the vessel wall (lllig 1961; Rhodin 1967, 1968; Hauck 1971; Westergaard 1974). A defmition of a...