Dr. J. S. Bolton has shown that in dementia the seat of greatest wasting of the cerebral cortex is commonly the prefrontal region, a region which (following Flechsig) he regards as a centre of higher association, the great development of which constitutes a leading character of the brain of man. This is a region which in man is one of the latest to myelinate (thirty-fifth in order, according to Flechsig), and if the order of myelination, as a part of ontogeny, may be taken as an approximate recapitulation of the order of phylogeny, then we might infer that the prefrontal region of man is a new region, of late development, absent or poorly developed in animals lower than man. Upon this assumption the predominant wasting of this region in dementia would be readily explained in accordance with Hughlings Jackson's doctrine of evolution and dissolution—dissolution following the inverse order of evolution. For an understanding of the pathology of insanity it is obviously important to inquire how far a study of comparative anatomy, especially of the primates, lends any support to such a conception. The aim of my discourse is accordingly to present, first, a brief survey of the recent observations of Dr. K. Brodmann, of Berlin, upon the comparative histology of the frontal cortex, and then some observations of my own upon naked-eye anatomy of the brain of man and higher apes.