a. Percent area of cortex supplied by MCA, ACA and PCA trees. 5% smaller MCA territory with aging; this favors smaller infarct volumes, opposite to Figure 6. b. MCA tree morphometry: similar 6% decrease in distal-most arterioles (DMA, both Type I and II) of MCA tree as 5% decrease in MCA territory. Collateral number decreased by similar absolute amount (though greater percentage) as DMAs, indicating concomitant loss of both. c, Type II DMAs are larger than Type I, consistent with greater flow of Type 2 to supply penetrating arterioles branching from collaterals. Aging decreases Type II diameter. This is consistent with decline in diameter of the collaterals they supply (Figure 3), which may relate to the mechanism of collateral loss (see Text). d, Collaterals not lost have same number of penetrating arterioles branching from them.