The modular organization of the pial arterial system in phylogeny.

We have investigated the pial arterial microcircles formed by the consecutive ramifications and anastomoses of minute pial vessels on the surface of the cerebral neocortex in various vertebrate species, representatives of various levels of phylogenic development. We found that the pial arterial microcircles became gradually more complex in the following order: hen less than rabbit less than cat less than dog less than monkey. The gradual development of microcircles involves various features in these species: They become formed predominantly of smaller pial arterial branches whose vasomotor activity is much higher as compared with that of the larger ones; each circle occupies a smaller area, and hence the number of circles per surface of neocortex increases; the density of arterial branches feeding the circles increases despite the smaller size of the latter; the number of off-shooting precortical and radial arteries penetrating the cortex increases; the areas of neocortex fed by individual radial arteries get gradually smaller. Thus, the development of the pial arterial bed contributes more and more toward precise spatial regulation of an adequate blood supply to increasingly smaller areas of neocortical tissue as we rise toward higher levels in phylogeny.