[*] Further studies have given evidence that the elimination of the acids resulting from energy-transformation as well as the conversion of energy stored in the kinetic organs causes histologic changes in the liver, the adrenals, and possibly in the brain. With these three kinetic organs we may surely associate also the "furnace," the muscles, in which the energy provided by the brain, adrenals, and liver, plus oxygen, is fabricated into heat and motion. Benedict, in his monumental work on metabolism, has demonstrated that in the normal state, at least, variations in the heart-beat parallel variations in metabolism. He and others have shown also that all the energy of the body, whether evidenced by heat or by motion, is produced in the muscles. In the muscles, then, we find the fourth vital link in the kinetic chain. The muscles move the body, circulate the blood, effect respiration, and govern the body temperature. They are the passive servants of the brain-adrenal-liver syndrome. Neither the brain, the adrenals, the liver, nor the muscles, however, nor all of these together, have the power to change the rate of the expenditure of energy; to make possible the increased expenditure in adolescence, in pregnancy, in courting, and mating, in infections. No one of these organs, nor all of them together, can act as a pace-maker or sensitizer. The brain acts immediately in response to the stimuli of the moment; the adrenals respond instantly to the fickle brain and the effects of their actions are fleeting; the liver contains fuel only and cannot activate, and the muscles in turn act as the great furnace in which the final transformation into available energy is made. The Thyroid Another organ--the thyroid--has the special power of governing the RATE OF DISCHARGE of energy; in other words, the thyroid is the pace-maker. Unfortunately, the thyroid cannot be studied to advantage either functionally or histologically, for there is as yet no available test for thyroid secretion in the blood as there is for adrenalin, and thyroid activity is not attended by striking histologic changes. Therefore the only laboratory studies which have been satisfactory thus far are those by which the iodin content of the thyroid has been established. Iodin is stored in the colloid lacunae of the thyroid and, in combination with certain proteins, is the active agent of the thyroid. Beebe has shown that electric stimulation of the nerve supply of the thyroid diminishes the amount of iodin which it contains, and it is known that in the hyperactive thyroid in Graves' disease the iodin content is diminished. The meagerness of laboratory studies, however, is amply compensated by the observations which the surgeon has been able to make on a vast scale--observations which are as definite as are the results of laboratory experiments.