"the theory that the mental powers of the individual consist of independent faculties, each of which has its seat in a different brain region, whose size is commensurate with the power of the faculty," 1815, literally "mental science," from phreno- "mind" (q.v.) + -logy "study of." Applied to the theory of mental faculties originated by Gall and Spurzheim that led to the 1840s mania for reading personality clues in the shape of a subject's skull and the "bumps" of the head. It was most popular from about 1810 to 1840. Related: Phrenological; phrenologist.
This theory, which originated at the close of the eighteenth century, assumes, moreover, as an essential part, the plasticity of the cranial envelop, by which the skull conforms externally, in the normal subject, to the shape and configuration of the brain within, so that its form and faculties may be determined, with sufficient exactness, from the skull itself, whether in the skeleton or in the living person. [Century Dictionary]