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, C,H,N,O, is a dehydrated form of kreatin, which is a normal constituent of urine. In watery solutions it is slowly converted into kreatin.

Allantoin, CH,N,O,, 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, CH10(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, C,HNO3, 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, CH,NSO,, is a constituent of one of the bile acids, and is also found in muscle juice. It may be regarded as amidoethyl-sulphonic acid.

Uric Acid, CH,N,O, (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:

IVE

CH,N,O. + H,O + 0 = C,H,N,O, + CO(NH,),. Hippuric Acid, C,H,NO,, 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 (C,H,O). In the body it is combined with bases, and is formed out of benzoic acid and glycin (amido-acetic acid), thus:

Benzoic Acid.

Hippuric Acid.

Water.

Glycin. C2H2(NH2)O.OH + C‚ÿO2 = C2H(CH ̧O)(NH)0.OH + H2O. By heating or putrefaction it is resolved into these constituents. Indol, CH,N, is produced in the intestinal canal by the putrefactive changes brought about by septic agencies during pancreatic digestion. It gives an odor to the fæces 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.

CLASS B.-NON-NITROGENOUS.

GROUP V.-CARBOHYDRATES.

Carbohydrates (general formula, CHO) are bodies in which the hydrogen and oxygen exist in the same proportion as in water, the carbon being variable. The following examples of this group are met with in the textures of the body:—

Grape Sugar (Dextrose), C8H12O6, occurs in minute quantities in the blood, chyle and lymph. It forms crystals which readily dissolve in their own weight of water. The watery solution has a dextro-rotatory power on the ray of polarized light. When mixed with yeast, the fungus (Saccharomyces cerevisia) of the

yeast causes alcoholic fermentation of the sugar, whereby alcohol and carbon dioxide are formed.

Moderate heat (25° C.) aids the process, and cold below 5° C. checks it; an excess of either sugar or alcohol stops it.

The presence of casein or other proteid material, when decomposing, gives rise to lactic fermentation, producing first lactic acid, then butyric acid, carbon dioxide and hydrogen.

Milk Sugar (Lactose), C12H22O11+ H2O, metameric with cane sugar (sucrose). It is the characteristic sugar found in milk. It is not so soluble as dextrose, and does not undergo direct alcoholic fermentation, but under the influence of certain organisms it readily gives rise to lactic acid by lactic fermentation in the same way as dextrose. (See page 102.)

Inosit, C6H12O6+2H2O, is an isomer of grape sugar, which is incapable of undergoing alcoholic fermentation. It is crystallizable, and easily soluble in water. It has no effect on the polarized ray. It is found in the muscles, and also in the lungs, spleen, liver and brain.

Glycogen, CH10O5, a body like dextrin, first found in the liver.. It gives an opalescent solution in water, and is readily converted into dextrose by an amylolytic ferment, or weak acids. It has a strong dextro-rotatory power. It can be found in most rapidly growing tissues. (See Glycogenic Function of the Liver.)

GROUP VI.-FATS.

These bodies have the same elements in their composition, but the hydrogen and oxygen have variable proportions—not that of water. Fats are found in large masses in some tissues, and also as fine particles suspended in many of the fluids. The fat of adipose tissue in man is a mixture of olein, palmitin and stearin, which are spoken of as the neutral fats.

The first is liquid, and the last two solid at normal temperatures, and the varying consistence of the fat of different animals

depends upon the relative proportions of the solid or liquid fats.

Fats are soluble in ether and chloroform, but quite insoluble in water. When agitated in water containing an albuminous body, and an alkaline carbonate in solution, fluid fat is broken up into small particles, which remain suspended in the liquid, forming an opaque milky emulsion.

Chemically, they are regarded as ethers derived from the triatomic alcohol glycerine, CH(OH), by replacing the OH group with the radicals of the fatty acids, thus:

Glycerine.

Palmitic Acid.

Tripalmitin

Water.

CH(OH), +3(C16H2O2) = C3H5 (C18H31O2)3 + 3H2O.

Under the influence of certain ferments they separate into glycerine and the fatty acid, uniting with the necessary elements of water.

When the neutral fats are boiled with alkaline solutions they are similarly decomposed, and uniting with the elements of water, form glycerine and fatty acids. The glycerine is thus set free, but the fatty acid combines with the alkaline metal to form a soluble soap. An insoluble soap may be obtained by substituting lead or lime, etc., for the alkali.

This splitting up of the neutral fats, stearin, palmitin and olein into sodium stearate, palmitate, or oleate goes on during digestion, and is said to be useful in aiding the absorption of fatty matters.

INORGANIC BODIES.

Water (HO) is present in nearly all tissues in larger proportion than any other compound, making up about 70 per cent. of the entire body weight. The amount in each texture varies, the different tissues having widely different consistence.

Water is introduced into the body not only as drink, but a large quantity is also taken with our solid food. It is highly probable that in the chemical changes which take place in the tissues, some water is formed by the oxidization of the hydrogen of the more complex substances.

In the economy it acts as the universal solvent in the fluids of

the body, and as the agent by means of which the chemical changes of the various organs can be accomplished.

Water leaves the body by the lungs as vapor, and by the skin, kidney, and many other glands, as the fluid in which the solids of their secretions are dissolved.

Inorganic acids occur in the body either combined, forming salts, in which condition we find several (sulphuric, phosphoric, silicic), or uncombined. In the latter state we have only two, viz.:

Hydrochloric Acid, HCl, which is formed in the stomach, and plays an important part in gastric digestion.

Carbonic Acid Gas, CO2, exists in most of the fluids of the body, having been absorbed by them from the tissues. The venous blood contains a considerable quantity, some of which is got rid of during the passage of the blood through the lungs. It is a waste product, which must be constantly eliminated from the body (see Respiration).

Salts.-A large number of salts occur in the tissues, generally in small quantity, in solution. In the teeth and in bone tissue salts exist in the solid form, and in much greater proportion than in any of the soft parts. Most of the salts are introduced into the economy with the food, but some, doubtless, are formed in the body itself. Our knowledge of the exact position occupied by the salts in the textures is very incomplete, as their amount is usually estimated from the ash of the tissue which remains after ignition, by which process some become altered, so that it is impossible to say what are the exact salts that are present in the body. They form chemical combinations with the complex organic compounds, which we do not understand, and probably have important functions to perform, such as rendering certain materials (globulins) soluble, or otherwise facilitating tissue change. The salts pass out of the body in many secretions, largely in the urine, where they influence the elimination of urea, and therefore form an important constituent of that secretion.

Common Salt (Sodium Chloride), NaCl, is the most widely distributed, and is present in greater quantity than any other salt