In this chapter we have examined MacLean’s contribution to two aspects of limbic brain anatomy and function: 1) This efforts at expanding the confines of the limbic brain to include a dispersed limbic system; and 2) his endeavor to place the mammalian limbic brain in an evolutionary context. Only 6 years after his limbic system paper expanding the limbic domain into the frontotemporal domain Walle Nauta would publish a paper extending the limbic system into the rostral brain stem. In the next chapter we will turn our attention to the further growth of the limbic system both caudally and rostrally, with particular emphasis on Nauta’s limbic midbrain.