implies the existence of _stimuli_ and
_susceptibility_ to stimulation. The stimulus may not be furnished
because the conditions on which it depends are wanting; again,
susceptibility may exist at one time and not at another. Stimuli and
susceptibility may be present in different degrees, but for the purpose
of healthful reproduction they must not be impaired. No single class of
foods, albuminous, starchy, saccharine, or mineral, is sufficient for
the nutrition of the body, but the food must contain substances
belonging to each of the different classes. If an animal be fed
exclusively upon albumen, though this substance constitutes the largest
part of the bodily mass, exhaustion will rapidly follow, since the food
does not contain all the essential, nutritive elements. Again, when the
solids of the body have been wasted, they lose their susceptibility to
stimuli, and the food does no good. Thus patients become emaciated
during acute attacks of disease, upon the cessation of which they are
too feeble to recover, simply because they have lost the power to digest
and assimilate their food.

In inanimate bodies, as in crystals, forces come to rest, but the very
idea of life implies action and continual change. Hence diversity of
constitutions and different temperaments are essential in order that
marriage may result in the reproduction of vigorous beings.

VITAL AND NON-VITAL TEMPERAMENTS.

[Illustration: Fig. 78.]

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 differenc