These, as has been stated before, are generally the outcome of the active chemical changes necessary for the growth and vitality of the living protoplasm, and are for the most part soon eliminated by the excretory glands, so that but small quantities of them can be found in the active tissues where they are produced.

Urea, CO(NH2)2, is the most important constituent of the urine of mammalia, but not of that of birds or reptiles. Traces of it may be found in the fluids and tissues of the body. It is readily soluble in water and alcohol, and forms crystals when its solution is concentrated. It decomposes when treated with some strong acids or alkalies, taking up water and yielding CO2 and NH3; and with nitrous acid gives C02 + N2 + 2(H20). It was the first of the so-called "organic" compounds to be made artificially, being obtained by Wohler in 1828 by mixing watery solutions of potassium cyanate and ammonium sulphate, evaporating to dryness and extracting with alcohol, or, in short, by heating ammonium cyanate, with which it is isomeric.

Ammonium Cyanate. Urea

NH1.CNO = CO.NH2.NH2. It can now be produced artificially in other ways.

It has also been considered a monamide of carbamic acid (CO.OH.NH2), a molecule of hydroxyl being replaced by one of amidogen, NH2, thus - CO.NH2.NH2. In the presence of septic agencies, in a watery solution, urea takes up two molecules of water and is converted into ammonium carbonate -

CO(NH2)2 + 2H20 = CO(ONH1)2.

The so-called alkaline fermentation of urine depends upon this change. The reader is referred to the Chapter on Excretions (xxii), where more complete information is given.

Kreatin, C4H9N302, occurs in muscle and many other textures.

It may be converted into kreatinin by the action of acids by simple dehydration. It can also be split up into sarcosin and urea.

Kreatinin, C4H7N30, is a dehydrated form of kreatin, which is a normal constituent of urine. In watery solutions it is slowly converted into kreatin.

Allantoin, C4H6N403, found in the allantoic fluid and the urine of the foetus and pregnant women. It is crystallizable, and is converted into urea and allantoic acid by oxidation.

Glycin (Glycocoll'or Glycocine), C2H2(NH2)O.OH, is regarded as amido-acetic acid. It does not occur free in the body, but enters into the composition of the bile acids and hippuric acid. It is soluble in water, and insoluble in cold alcohol and in ether.

Leucin, C6H10(NH2)O.OH, or amido-caproic acid, is found in the secretion of the pancreas and some other glands. It is one of the principal products of the decomposition of albuminous bodies, from which it can be obtained by boiling with sulphuric acid, in the form of peculiar rounded crystals.

Tyrosin, C9H11N03, though belonging to a distinct chemical series (aromatic), is only found in company with leucin in the decomposition of albuminous bodies, and normally in the pancreatic secretion. Its constitution is said to give warranty for the name oxy-phenyl-amido-propionic acid.

Taurin, C2H7NS03, is a constituent of one of the bile acids, and is also found in muscle juice. It may be regarded as amido-ethyl-sulphonic acid.

Uric Acid, C5H1N403 (dibasic), is found in large quantities in the excrement of birds and reptiles, but in a small and variable quantity in the urine of man. Traces have been found in many tissues, in some of which quantities accumulate as the result of pathological processes (gout). It forms salts which are much less soluble in cold than in hot water, and make the common sediment in urine. The acid salts are less soluble than the neutral. The common test for uric acid consists of slowly evaporating the substance to dryness with a little nitric acid, and to the residue adding ammonia, when a bright purple color is produced (murexide test). Uric acid is supposed to be a step in the production of urea, which is one of the results of its oxidation in the presence of acids, thus: -

Uric Acid. Alloxan. Urea

C5H1N403 + H20 + O = C4H2N204 + CO(NH2)2.

Hippuric Acid, C9H9N03, occurs in considerable quantities in the urine of the horse and herbivora generally. It is found but very sparingly in man's urine, but it appears in large quantities after benzoic acid and some other medicaments have been taken. In constitution it is an amido-acetic acid in which one atom of the hydrogen is replaced by the radical benzoyl (C7H50). In the body it is combined with bases, and is formed out of benzoic acid and glycin (amido-acetic acid), thus: -

Glycin. Benzoic Acid. Hippuric Acid. Water

C2H2(NH2)O.OH + C7H602 = C2H(C7H50) (NH2)O.OH + H20.

By heating or putrefaction it is resolved into these constituents.

Indol, C8H7N, is produced in the intestinal canal by the putrefactive changes brought about by septic agencies during pancreatic digestion. It gives an odor to the faeces and a red color with nitrous acid.

Indican, a peculiar substance sometimes found in the urine and sweat. With oxidizing agents it yields indigo blue. By this fact it is easily recognized. An equal volume of hydrochloric acid and a very small quantity of calcium hypochlorite (bleaching lime) is added, and the indigo which is formed can then be dissolved and separated by agitation with chloroform.