286 LIFE : OUTLINES OF GENERAL BIOLOGY 



To understand this even a little, it is necessary to proceed 

 patiently and to state a few chemical facts. The proteins of our 

 food, such as the casein of cheese, the gluten of wheat, the vitellin 

 of yolk of egg, are nitrogenous carbon compounds, and they are 

 digested in the food-canal into the simpler amino-acids, which are 

 absorbed into the blood. In part these go to repair the wear and 

 tear or to furnish material for growth; but they are also in great 

 part broken down into ammonia, which is apt to be poisonous, 

 and some derivative of a fatty acid. The latter is burnt up to supply 

 bodily energy; the former is converted into urea, and this is in 

 great measure the work of the hard-worked liver. What the liver 

 prepares for excretion, such as urea in mammals, is filtered out by 

 the kidneys. The preliminary process of splitting off the ammonia 

 and forming a fatty acid is called de-amination. The second step 

 is the liver's conversion of ammonia into urea. The third step is 

 the actual elimination effected by the kidneys. 



It is of theoretical interest to distinguish the endogenous chemical 

 routine by which proteins, changed into amino-acids, are used to 

 furnish material for growth and for repair, from the exogenous 

 metabolism by which amino-acids are de-aminated to supply 

 energy by the burning up of the resulting fatty acid. Along 

 both lines the same final result may be reached, namely urea. 

 But it is practically important, for health reasons, to realise that 

 the nitrogenous waste-products that are filtered out may be the 

 ashes of the living fire, or may be the direct and indirect results 

 of the wear and tear of tissues. It may seem absurd to talk of a 

 fluid as "ashes", but the various states of matter readily pass 

 into one another, and everyone who has kept birds knows that 

 the nitrogenous waste-products from the kidneys are semi-solid. 

 They form most of the guano of the crowded bird islands of 

 Peru, for the imdigested residue from the food-canal is much 

 less important. 



Two points stand out prominently, one practical and the other 

 theoretical. The practical point is the danger of giving liver and 

 kidneys too much profitless work to do by taking in far more nitro- 

 genous food than is needed by the requirements of the body either 

 for repair or for energy. The theoretical point is that while urea 

 is the commonest nitrogenous waste-product in mammals, amphi- 

 bians, and fishes, and uric acid is characteristic of birds and reptiles, 

 there are other kinds of ashes, such as salts of ammonia, in some 

 backboneless animals. It is a question of high biological interest 

 whether skeleton-making in animals like sea-urchins and moUuscs, 

 and pigment-making in others like butterflies, may not express 

 profitably regularised ways of dealing with the ashes of life's fire. 



So far, a mere outHne of the animal's method of dealing with 

 nitrogenous waste-products — a problem that has been much 



