The isolation of thyroxin has made possible the determination of the influence of the thyroid hormone upon the evolution of energy in any higher animal organism. There is, for every individual, a constant, known as the metabolic rate, or the combustion rate, a reading of the rate at which his cells are consuming material for heat. The metabolic rate is thus a gauge of the energy pressure within the organism. It may be calculated by measuring the amount of carbon dioxide gas exhaled during a unit of time, and the number of calories of heat radiated by the skin simultaneously. A simplified device has lately rendered it practicable to make actual determinations by a few five-minute readings of the rate of oxygen absorption by the lungs. Plummer, also connected with the Mayo Foundation, has shown that what would amount to less than a grain of the thyroxin would more than double the amount of energy produced in a unit of time. To be exact, one milligram of thyroxin increases the metabolic rate two per cent. That illustrates some of the power of the internal secretion of the thyroid and its importance to normal life. THE MOBILIZATION OF ENERGY But not only is the height of pressure of energy in the cells controlled by the thyroid. The mobility of that energy is also controlled. Without it, rapid and large fluctuations of energy output, and elasticity and flexibility of energy mobilization for any sudden mental or muscular act, let alone an emergency, become impossible. A woman suffering with myxedema, the condition described by the English physician Gull as a cretinoid state supervening in the adult life of woman, has an insufficient amount of thyroxin in her blood and tissues. She is clumsy and awkward and will stumble when endeavoring to walk upstairs. Any effort is almost paralyzed because the range of fluctuation of energy, the ability to mobilize energy, in turn dependent upon an ability to increase the metabolic rate, is limited. In slang phrase, she cannot step on it. Her existence is set to go at a rate in the neighborhood of forty per cent below the normal. By the administration of thyroxin, her metabolic rate can be raised to any desired figure, the spark can be adjusted, so to speak, to any point we like, and it can be so maintained for years.