Flesh Bases and Waste Products.--As the result of destructive metamorphosis or the wearing out of the body, there remain certain waste products which have to be expelled as soon as is possible. Their retention and accumulation would soon produce death. A part is expelled by the lungs as carbon-dioxide, or as it is generally though less correctly termed, carbonic acid. Upon the breaking down of the complex proteid and other nitrogenous matter, the nitrogen is left in comparatively simple combinations. These effete nitrogen compounds are commonly termed flesh bases or nitrogenous extractives. They exist in small quantity in flesh meat, but are concentrated and conserved in the making of beef-tea or beef-extract. The spleen, lymphatic and other glands, and especially the liver, break these down into still simpler compounds, so that the kidneys may readily separate them from the blood, that they may pass out of the body. By far the largest part of this waste nitrogen is expelled from the bodies of men and many other mammals in the form of urea. Pure urea is an odourless transparent crystalline substance, of cooling saline taste like nitre. It is soluble in an equal volume of water, and is expelled from the body with great ease. In the herbivora the nitrogenous waste takes the form of another body called hippuric acid. The nearly solid light-coloured urinary excretion of birds and serpents consists of urates; this is uric acid in combination with alkalies. In man, in addition to the urea excreted, there is also a little hippuric and uric acid or compounds of these. Uric acid is a transparent colourless crystalline body almost insoluble in water but soluble as urates in the presence of alkalies. As deposited from urine it is of a dull red sand-like appearance, as it has a great affinity for any colouring matter that is present. It is only possible to make a brief reference to the chief organic bases. The xanthine bases are closely related to uric acid. Some of these occur in small quantity in the urine and animal tissues, others, such as caffeine, occur in plants. Creatine is a constant constituent of muscle substance. In fowl's flesh there is said to be 0.32 per cent., in cod-fish 0.17 per cent., and in beef 0.07 per cent. Creatinine is produced from creatine with great facility; it exists in urine. Both creatine and creatinine are readily soluble in water. A series of bases, closely allied to creatinine have been isolated from the flesh of large animals by A. Gautier; they are known as Gautier's flesh bases. When administered to animals, these act more or less powerfully on the nerve centres, inducing sleep and in some cases causing vomiting and purging in a manner similar to the alkaloids of snake venom, but less powerfully than the ptomaines. These bases are formed during life as a result of normal vital processes and are termed leucomaines.