In the preceding chapter, we attempted to illustrate the unique blending of mind and body by means of the nervous system, and we now propose to exemplify the physical conditions of the organism by certain correspondences, observed in the development and conditions of that system. If nature answer to mind in physical correspondences, she will observe the same regularity in physical development. The simplest classification of the temperaments is represented in Fig. 78. Not only is mental activity dependent upon a vital activity in the brain, but the development of the cerebrum is dependent upon the supply of blood. The growth of the intellect requires the same conditions that aided in the development of Vulcan's right arm: waste and supply; disintegration and reparation of tissue. Our modern iron forges produce many an artisan whose great right arm proclaims him to be a son of power as well as of fire. Thus the fervid intellect, while forging out its thoughts, increases in size and strength. The difference between the development of the two is this; that the exercise of the blacksmith's right arm quickens the activities of all the bodily functions, whereas the employment of the intellect does not offer any healthy equivalent. Physical exercise is a hygienic demand, but intellectual employment exerts no salutary influence on the body, while it is constantly expending the nutritive energies of the blood. The emotions, likewise, make exhaustive draughts upon nutrition to supply the waste of brain substance, just as certainly as physical labor causes muscular change, and demands reparation. One expends cerebral, the other, muscular substance. The one is healthful in its general tendencies, the other, comparatively wasteful and destructive. / DISINTEGRATING, The intellectual faculties are { EXPENDING, \ DERIVING. / ENGROSSING, The emotive faculties are { EXHAUSTING, \ DEVITALIZING. These nervous forces are transformed into spiritual products.