That muscular work is performed by energy supplied in carbonaceous material is shown by the great increase of carbon-dioxide output in severe muscular work, which may exceed twenty times the output during rest. Furthermore, the storage of glycogen in muscle, and the disappearance of this glycogen deposit from excised muscle stimulated to activity,[122] or its reduction after excessive contractions produced by strychnine,[123] and the lessened ability of muscles to work if their glycogen store has been reduced,[124] and the simple chemical relation between sugar and the lactic acid which appears when muscles are repeatedly made to contract, are all indications that carbohydrate (sugar and glycogen) is the elective source of energy for contraction. This conclusion is supported in recent careful studies by Benedict and Cathcart,[125] who have shown that a small but distinct increase in the ratio between the carbon-dioxide breathed out and the oxygen breathed in during a given period (the respiratory quotient) occurs during muscular work, and that a decrease in the quotient follows, thus pointing to a larger proportion of carbohydrate burned during muscular work than before or after--i. e., a call on the carbohydrate deposits of the body. Whether circulating sugar can be immediately utilized by active muscles has been a subject of dispute. The claim of Chauveau and Kaufmann[126] that a muscle uses about three and a half times as much blood sugar when active as when resting, although supported by Quinquaud,[127] and by Morat and Dufourt,[128] has been denied by Pavy,[129] who failed to find any difference between the sugar content of arterial and venous blood when the muscle was contracting; and also by Magnus-Levy,[130] who has estimated that the amount of change in sugar content of the blood passing through a muscle must be so slight as to be within the limits of the error of analysis. On the other hand, when blood or Ringer’s solution is repeatedly perfused through contracting heart muscle, the evidence is clear that the contained sugar may more or less completely disappear. Thus Locke and Rosenheim[131] found that from 5 to 10 centigrams of dextrose disappeared from Ringer’s solution repeatedly circulated through the rabbit heart for eight or nine hours. And recently Patterson and Starling[132] have shown that if blood is perfused repeatedly through a heart-lung preparation for three or four hours, and the heart is continually stimulated by adrenin added to the blood, the sugar in the blood wholly vanishes; or if the supply of sugar is maintained, the consumption may rise as high as 8 milligrams per gram of heart muscle per hour--about four times the usual consumption. When an animal is eviscerated it may be regarded as a preparation in which the muscles are perfused with their proper blood, pumped by the heart and oxygenated by the lungs. Under these circumstances, the percentage of sugar in the