Fig. 11.4-5. Transverse section of cactus stem (Morawetzia doelziana; no common name). The low magnification micrograph shows the pith (and a little bit of the surrounding wood on the right). There are numerous vascular bundles in the pith (vascular bundles in the pith are known as medullary bundles), but at this low magnification they are difficult to see. The high magnification micrograph shows one medullary bundle: it is collateral, having both xylem and phloem.

Most cacti have both cortical bundles and medullary bundles. And of course they all have the ordinary bundles that make up the central ring of vascular bundles. Consequently, their stems have three sets of vascular tissues, and they are all interconnected: medullary bundles and cortical bundles both attach to the central ring of bundles, and medullary bundles can run through the parenchyma between the main bundles and connect directly with cortical bundles. By having these two extra sets of vascular tissues, it is feasible for the cortex and pith of cacti to evolve to be much larger than the corresponding tissues in ordinary non-succulent plants.
For more information on medullary bundles, see:
Mauseth, J. D. 1993. Medullary bundles and the evolution of cacti. American Journal of Botany 80: 928-932.