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 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 com

position 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 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.

6. In some diseases which interfere with or suppress the secretion of the kidneys, an accumulation in the blood of certain poisonous or injurious materials takes place, and gives rise to the gravest symptoms called uræmic poisoning, which closely coincide with those observed in experimental annihilation of the renal function.

From the foregoing it would appear to be satisfactorily settled that the urea, which is by far the most important ingredient of the secretion of the kidney, is probably made elsewhere and not in that organ, whose duty seems to be chiefly to remove it from the blood. This is most probably also true of all the other organic constituents of the urine. The question then arises, where is the urea formed?

We naturally turn for an answer to the most widespread and most actively changing nitrogenous tissue, namely, muscle. Here we find only a partial explanation of the source of urea, for neither does muscle contain much urea, nor does active muscular work perceptibly increase the general urea elimination. In muscle, however, a material closely allied to and readily convertible into urea, namely, kreatin, occurs, and it has been suggested that this substance is changed into ured in the kidney. This cannot explain the origin of all the urea, for, as already remarked, the amount of urea excreted does not correspond with the muscle metabolism.

A considerable quantity of urea no doubt comes from muscle, which tissue forms so large a part of our bodies, but we must conclude that there are many other sources of urea, because there are many other organs where nitrogenous substances are undergoing chemical changes and gradual waste.

The liver is specially worthy of note as a source of urea, since it helps to explain the striking relation between the amount of albuminous food and the quantity of urea eliminated. There can be no doubt that most people consume much more albuminous food than is necessary for the adequate nutrition and preservation of the nitrogenous tissues, and therefore must have a surplus of nitrogenous material. It may be remembered, as was pointed out in the chapter on Digestion, that in all parts of the aliment

ary tract there is a limit to the absorption of peptones, and that in the small intestine when delay in absorption occurs the decomposition of peptones results, because in prolonged pancreatic digestion these peptones are changed into leucin (CH1NO,) and tyrosin (CHNO3), and as such pass into the portal circulation to be borne to the liver. In the liver it is highly probable that these bodies are converted into urea, for, when they are introduced into the intestinal tract, they are absorbed and an excess of urea appears in the urine. Thus the surplus of the proteid food, before it really enters the system, is broken up in the intestine into bodies which, notwithstanding the difficulty of explaining the chemical process, may be regarded as a step toward the formation of urea. This view is supported by the facts that (1) in disease of the liver tyrosin and leucin appear in the urine; (2) if these bodies be introduced into the general circulation, by the jugular instead of the portal vein, they are excreted unchanged by the kidneys.

NERVOUS MECHANISM OF THE URINARY SECRETION.

With regard to the influence exerted by the nervous system on the renal secretion, we have but little satisfactory information, although there can be no doubt that here, as in other glands, the process is under the control of the nerves. Many of the circumstances which cause greater activity of secretion, such as taking large quantities of water, etc., have no effect on the general blood pressure, so that, if the increased flow be brought about by the vasomotor mechanisms, it must be by means of nervous channels altering the blood flow in the special arteries of the glands. Further, some emotional conditions exist, such as hysteria, in which an unaccountably great quantity of urine of very low specific gravity is evacuated.

With regard to the effects of the vasomotor nerves, we know that section of all the nervous twigs going to the kidneys causes great congestion and an immense increase in the secretion, which commonly contains albumin. This no doubt depends on the sudden rise in pressure in the glomeruli, owing to the dilatation of the arterioles. If the splanchnies, in which the renal vaso