Now of all the different factors which determine the tones, that is to say, the internal pressures, of the various parts of the vegetative apparatus (including all structures not controlled by the will in the term), the internal secretions or hormones are by far the most important. This significance is conferred upon them because it is by their activities primarily that these pressures are produced, regulated, lowered and heightened; in short, controlled. We have seen how the thyroid and adrenal hold the reins of the drive or check systems in the vegetative apparatus. Together with the other ductless glands, they decide the advance or halt, forward or retreat, tension or relaxation, charge and discharge, of the visceral--involuntary muscle--blood vessel combination which is at the core of life. Here again they emerge as the directorate. Carlson, the Chicago physiologist, who probably knows more about being hungry than any other man on the planet, once demonstrated that the injection of an ounce or two of the blood, which means the internal secretion mixture, of a starving animal, into one not starving increased the signs of hunger and the accompanying hunger contractions of the stomach. There can be no doubt that hunger is the expression of a certain specific concentration of internal secretion or secretions in the blood. When the quantity, in the cycles of metabolism, becomes sufficiently great, it stimulates the stomach to contract in a way which augments the pressure within it to a point at which the feeling of hungriness, and the wish to satisfy it, or to get rid of it, becomes imperative, and the dominant of consciousness.