Every individual is susceptible to impressions which dispose to action. Impressions which excite or increase this disposition, are called _stimuli_. Vital change 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.]