Figure 7.

Elementary loops. A, An elementary loop through a branch within the network mesh is defined as the shortest cycle in a graph that contains that branch. B, Distribution of the number of nodes/branches in an elementary loop. C, Loop-length distributions for the entire spine set, shown separately for the head and neck regions. D, A principal component analysis dissects the spatial arrangement of nodes inside an elementary loop. The first two components account for 95% of the total variance, which implies that elementary loops are predominantly planar. E, The first two principal components plotted against each other, shown as a density plot for all loops in the set. The dashed line corresponds to perfect planarity, i.e., vanishing third principal component.