The metabolism of the body is not normally derived from the oxidation of nitrogenous foods. Failing a sufficient supply of other kinds of food, they may be used as sources of energy; but we must picture them as splitting into carbonaceous and nitrogenous portions. If, after the reserve of glycogen in the liver has been brought low by abstention from carbohydrates and fats, nitrogenous food is consumed, and the muscles are then called upon to do severe work, the amount of carbonic acid and water given off rises at once. The excess of urea derived from the nitrogenous food which was destroyed for the purpose of liberating the energy which the muscles expended makes its appearance some time later. If the diet contains a sufficiency of carbohydrates, muscular work does not increase urea. The output of urea is exceedingly steady. It is not increased by muscular work, nor diminished, beyond a certain limit, by absence of food. The tissues are constantly throwing off nitrogen-containing molecules, which, if the body is not to waste, must be as constantly renewed. =Uric Acid.=—When nitrogenous metabolism has reached the bottom, when albuminous substances have been shaken into the simplest and most stable compound or compounds which the muscles are capable of making (we know not whether the end-products be one or many), they are carried to the liver by the blood. The mammalian liver converts them into urea; the liver of birds and reptiles changes them into uric acid. Uric acid is not, however, completely absent from the urine of carnivorous animals. In Man the amount excreted is about 0·8 gramme per diem, but subject, even in perfect health, to considerable variations (0·2 gramme to 1·4 gramme). There is no reason for thinking that uric acid is made in the liver of mammals. On the contrary, it seems to be either an end-product of the disintegration and oxidation of leucocytes (_cf._ p. 53), or, like certain other more complex nitrogenous compounds which appear in very small quantities in the urine, the relic of albuminous food which has missed the broad down-path, via muscles and liver, to the kidney. It is a troublesome burden for lymph and blood, and, unfortunately, the kidney finds difficulty in throwing it out. Uric acid has a pernicious way of accumulating in tissue-spaces, producing all the malevolent symptoms of gout. During an acute attack of gout the quantity of uric acid in the system may be largely increased. It may be so abundant in the blood that, when a sample is allowed to cool, uric acid begins almost immediately to crystallize out. Speaking generally, it is right to ascribe gout to an over-production of uric acid; but it must be remembered that the balance between elimination and production is very delicately adjusted. During an attack of gout the amount excreted in the urine is not increased; frequently it is less than usual. The clearing up of the attack is accompanied by abundant