We now approach the subject of the several particular mental qualities, and the groups thereof, both in the phase of their inner states and that of their outer form. In the consideration of both of these phases we must avail ourselves of the investigations and researches of the old phrenologists who cleared a path for all who follow. Although many of the phrenological theories are rejected by modern psychologists and biologists, nevertheless their work established a firm foundation for the science of the study of the brain and its functions. And to Gall and his followers we are indebted for the discovery and teaching that the activity and development of the several mental qualities or faculties manifest in outer form in the shape of the skull. [Illustration: FIG. 1 THE MENTAL QUALITIES] The general principles of phrenology may be briefly stated as follows: I. The Brain is the organ of the mind. II. The mind is not a single entity or power, but has several faculties, stronger or weaker, which determine the character of the individual. III. That each faculty or propensity has a special organ in the brain. IV. The size of the brain (the quality being equal) is the true measure of power. V. There are several groups of faculties, and each group is represented by organs located in the same region of the brain. VI. The relative size of each organ results from the activity of its appropriate faculty. VII. The size of the organ is indicated by the appearance and size of the skull immediately over the region of the organ. VIII. The Quality and Temperament of the organization determine the degree of vigor, activity, and endurance of the mental powers. Modern psychology and biology claim to have disproven many of the phrenological contentions, while other lines of investigation have given us other theories to account for the phenomena first noted by the phrenologists. Some investigators of brain development and action hold that while certain mental states manifest in outer form on portions of the skull, the phenomenon is due to the action of the _cranial muscles_ rather than to the fact of the localization of special faculties--that each mental state is associated with certain actions on the part of certain cranial muscles which in turn exert a modifying effect upon the shape and size of the skull.