The course and direction of persistent foetal venous channels are easily defined in the relatively simple segmental portions of the nervous system, such as the spinal cord, the myelencephalon, the metencephalon, and the mesencephalon. In sites were embryonic development is more complicated, like the diencephalon and the telencephalon, rapid growth and folding of the cortex complicate the patterns of the foetal blood vessels. Arteriovenous malformations in such sites are correspondingly more complex.