Hippuric acid (C,H,NO,) is a normal constituent of human urine, occurring, however, in very small quantities. On the other hand, it is one of the most important nitrogenous constituents of the urine of the herbivora, where it takes the place of uric acid. Its presence depends on the existence of certain ingredients (benzoic acid, etc.) in the food, which are capable of combining with glycin, and forming a conjugated acid, a molecule of water being formed at the same time, thus

11=

+

Water.

CHO2+C,H,NO, C,H,NO, H2O.

6 2

The amount of hippuric acid increases with increased consumption of vegetable food, in the cellulose of which the materials. exist that are required for its formation. The union of glycin and benzoic acid may take place in the liver, for, after removal of that organ, benzoic acid injected into the veins appears unchanged in the urine; but the extirpated kidney is also said to be capable of effecting this synthesis.

Oxalic acid (CH,O,) occurs often, but not constantly, in the urine. It is generally united with lime. It is said to appear in greater quantity, together with an excess of uric acid, after meals, and therefore to be related to the production of the latter in the body; but it probably is chiefly derived from oxalates contained in some materials taken with the food.

COLORING MATTERS.

It appears probable that the color of the urine depends on the presence of small quantities of distinct substances which have different origins in the body. Three such have been described, and may be taken provisionally to represent our knowledge of the subject:

1. Urobilin, which is an outcome of the coloring matter of the bile, and therefore a remote derivative of the coloring matter of the blood, is frequently present in the urine. It is probably the same as hydrobilirubin, some of which is occasionally absorbed from the intestinal tract and eliminated by the kid

neys.

2. Urochrome is said to be the special pigment of the urine. It

oxidizes on exposure, forming a reddish substance that gives the dark color to some urinary sediments (Uroerythrin).

3. A certain material (Indican) capable of producing Indigo, is commonly present in the urine of man, and in greater quantity in that of some animals, particularly the horse. It is supposed to be formed from the indol that arises from the putrefactive changes consequent on the pancreatic digestion. The indol is absorbed and unites with sulphuric acid to form Indican, which is a yellow substance. Under certain conditions it can be converted by oxidation into indigo-blue.

INORGANIC SALTS.

The urine is the great outlet for all inorganic salts. important of these are—

The most

Common salt (NaCl), of which a very variable but always considerable amount passes away in the urine. The average quantity excreted per diem may be said to be about half an oz. (15 grammes). It depends greatly on the quantity taken with the food, and falls during starvation, but does not completely disappear. It is said that if absolutely no common salt be taken with the food the quantity of NaCl excreted diminishes greatly, and albumin appears in the urine about the third day. The amount of salt eliminated follows, with striking accuracy, the changes that take place, at different times and under different circumstances, in the quantity of urea excreted. These facts seem to indicate that there is some relationship between the secretion of the two bodies, or that sodium chloride participates in the chemical changes of the nitrogenous tissues. In many diseases there occur variations in the quantity of common salt in the urine which can hardly be explained by the change in or absence of food.

Phosphates.-About 60 grains (3 to 4 grammes) of phosphoric acid is excreted daily in the urine, being combined with alkalies to form salts, viz., potassium, sodium, calcium, and magnesium phosphates.

Sulphates.-Nearly 40 grains (2 to 3 grammes) of sulphuric acid, as sulphates of alkalies, are daily got rid of in the urine.

The acid comes partly from the food, but chiefly from the oxidation of the sulphur contained in the proteids of the tissues.

A considerable quantity of potassium, sodium, calcium, and magnesium, combined as already mentioned, or with chlorine, is contained in the urine.

Small traces of iron are also always present in the urine.

Gases. The urine also contains free CO2, N, and some O. 100 volumes of gas pumped out of fresh urine have been found to consist of

Different kinds of substances occur in urine under circumstances of special physiological interest, and therefore may be here enumerated, although their accurate study belongs rather to pathology. First among these to be named is

Albumin, which occurs from (1) any great increase in the blood pressure in the renal vessels, whether caused by increased inflow or impeded outflow. (2) Excess of albumin in the blood, and, strange to say, some forms of albumin escape much more readily than others. Thus, egg albumin, globulin, or peptone, if introduced artificially into the blood, are soon found in the urine. (3) A watery condition of the blood, such as would give rise to oedema elsewhere. (4) Total abstinence from NaCl for some time. (5) Destruction of some of the epithelium of the urinary tubes. Next in importance to albumin are the following :

Grape sugar, of which normally only the merest trace occurs in the urine, although there is always a certain quantity in the blood. It is present in large quantities in (1) the disease known as diabetes, when a great quantity of pale urine with a very high specific gravity is passed. (2) After injury of a certain part of the floor of the 4th ventricle of the brain. (3) After poisoning by curara, carbonic oxide, and nitrate of amyl. In short, any disturbance of the circulation of the liver gives rise to an increase of sugar in the blood, and when the amount reaches 6 per cent. it appears in the urine.

Bile Acids and Pigments appear in the urine when, from occlusion of the bile ducts, they find their way into the blood.

Leucin and Tyrosin also occur in the urine, but only after interference with the functions of the liver.

The urine undergoes important changes after being voided, the explanation of which is of much interest to the practitioner, and must be understood by the student of medicine. The urine often loses its transparency as soon as it gets cold, though perfectly clear when passed, or when again heated to the body temperature, for the urates are soluble in warm but almost insoluble in cold water. The "muddiness," which soon settles down, as a more or less brightly-colored sediment, is chiefly caused by the precipitation of acid sodium urate, stained with a coloring matter derived from the urochrome. When this occurs the urine will always be found to be distinctly acid, and if it be left standing for some time in a cool place, the acidity will be found to increase, owing to the presence of a peculiar fungus which sets up acid fermentation. This is said to depend on the formation of lactic and acetic acids, and crystals of uric acid, amorphous sodium urate, and crystals of lime oxalate are deposited.

After a certain time (which is shorter when the urine is not very acid and is exposed to a warm atmosphere) the development of bacteria occurs in it, and causes the urea to unite with water and to change in the manner already mentioned (p. 75) into ammonium carbonate. This gradually neutralizes the acidity, and finally renders the urine alkaline. At the same time an amorphous precipitate of lime phosphate appears, and crystals of ammonio-magnesium phosphate and of ammonium urate are produced.

URINARY CALCULI.

Various ingredients of the urine, which are difficult of solution, sometimes become massed together as concretions, particularly if there exist any small foreign body in the bladder, which, by acting as a nucleus, lays the foundation of a stone. Sometimes small concretions are formed in the tubes or pelvic recesses of the kidney, and, when these make their way into the bladder, they commonly grow larger and larger. The structure and composi

tion of a calculus often gives the history of its own transit from the kidney, and also of various changes in the metabolism of the individual, for successive layers of different substances are generally found in a stone that has attained any great size. The chief materials found in calculi are-uric acid, ammonium urate, calcium oxalate and carbonate, ammonio-magnesium phosphate, etc.

SOURCE OF UREA, ETC.

The question as to whether the chief materials of the urine preexist in the blood and are therefore merely removed by the kidney, or are manufactured by the special powers of the renal cells, has been widely discussed, and though the great weight of evidence is in favor of the former view, some of the experimental results on the subject are rather conflicting.

The following are the more important points in the argu

ment:

1. The blood normally contains most of the important substances found in the urine; so they need not necessarily be made in the kidney.

2. The blood in the vessel leading to the kidney-the renal artery is said to contain more urea than that in the vessel leading from it-the renal vein-so that the blood appears to lose urea in passing through the kidney.

3. If the ureters be tied and the elimination be thus prevented, urea accumulates in the blood. This can hardly be made by the kidney, because—

4. If the renal arteries be tied so that no blood goes to the kidneys to affect the elaboration of urea in those organs, then the same accumulation results, showing that the kidneys are certainly not the only organs where urea is made.

5. Extirpation of the kidneys also gives rise to a great increase of the urea in the blood. The amount of urea in the blood after nephrotomy is said to increase steadily with the time which elapses after the operation, and the amount accumulated corresponds to the amount that would normally have been excreted in the same time, had the animal not been operated upon.