Detailed measurements of vessel lengths and diameters from the coronary network of the human heart are presented. To allow accurate definition of length and diameter, the measurements were made in terms of vessel segments rather than whole vessels. A vessel segment was defined to be the interval between two consecutive branching sites. The results provide the first quantitative description of the coronary network. The new concept of vessel segment and the method of analysis are proposed as a means for (a) an accurate description of the branching characteristics of the coronary network, (b) comparing the network with others in the cardiovascular system and (c) comparing the vasculature in the normal versus that in the diseased heart.