generally, to take some direct and essential part in the vital operations. Many of them, like the gastric and pancreatic juices, are reabsorbed after they have done their work, and again enter the current of the circulation. But the urine is merely a solution of excrementitious substances. Its materials exist beforehand in the circulation, and are simply drained away by the kidneys from the blood. There is a wide difference, accordingly, between the action of the kidneys and that of the true glandular organs, in which certain new and peculiar substances are produced by the action of the glandular tissue. The kidneys, on the contrary, do not secrete anything, properly speaking, and are not, therefore, glands. In their mode of action, so far as regards the excretory function, they have more resemblance to the lungs than to any other of the internal organs. But this resemblance is not complete; since the lungs perform a double function, absorbing oxygen at the same time that they exhale carbonic acid. The kidneys alone are purely excretory in their office. The urine is not intended to fulfil any function, mechanical, chemical, or otherwise; but is des tined only to be eliminated and expelled. Since it possesses so peculiar and important a character, it will require to be carefully studied in detail.

The urine is a clear, watery, amber-colored fluid, with a distinct acid reaction. It has, while still warm, a peculiar odor, which disappears more or less completely on cooling, and returns when the urine is gently heated. The ordinary quantity of urine discharged daily by a healthy adult is about 3xxxv, and its mean specific gravity, 1024. Both its total quantity, however, and its mean specific gravity are liable to vary somewhat from day to day, owing to the different proportion of water and solid ingredients entering into its constitution. Ordinarily the water of the urine is more than sufficient to hold all its solid matters in solution; and its proportion may therefore be diminished by accidental causes without the urine becoming turbid by the formation of a deposit. Under such circumstances, it merely becomes deeper in color, and of a higher specific gravity. Thus, if a smaller quantity of water than usual be taken into the system with the drink, or if the fluid exhalations from the lungs and skin, or the intestinal discharges, be increased, a smaller quantity of water will necessarily pass off by the kidneys; and the urine will be diminished in quantity, while its specific gravity is increased. We have observed the urine to be

reduced in this way to eighteen or twenty ounces per day, its specific gravity rising at the same time to 1030. On the other hand, if the fluid ingesta be unusually abundant, or if the perspiration be diminished, the surplus quantity of water will pass off by the kidneys; so that the amount of urine in twenty-four hours may be increased to forty-five or forty-six ounces, and its specific gravity reduced at the same time to 1020 or even 1017. Under these conditions the total amount of solid matter discharged daily remains about the same. The changes above mentioned depend simply upon the fluctuating quantity of water, which may pass off by the kidneys in larger or smaller quantity, according to accidental circumstances. In these purely normal or physiological variations, therefore, the entire quantity of the urine and its mean specific gravity vary always in an inverse direction with each other; the former increasing while the latter diminishes, and vice versa. If, however, it should be found that both the quantity and specific gravity of the urine were increased or diminished at the same time, or if either one were increased or diminished while the other remained stationary, such an alteration would show an actual change in the total amount of solid ingredients, and would indicate an unnatural and pathological condition. This actually takes place in many forms of disease.

The amount of variation in the quantity of water, even, may be so great as to constitute by itself a pathological condition. Thus, in hysterical attacks there is sometimes a very abundant flow of limpid, nearly colorless urine, with a specific gravity not over 1005 or 1006. On the other hand, in the onset of febrile attacks, the quantity of water is often so much diminished that it is no longer sufficient to retain in solution all the solid ingredients of the urine, and the urate of soda is thrown down, after cooling, as a fine red or yellowish sediment. So long, however, as the variation is confined within strictly physiological limits, all the solid ingredients are held in solution, and the urine remains clear.

There is also, in a state of health, a diurnal variation of the urine, both in regard to its specific gravity and its degree of acidity. The urine is generally discharged from the bladder five or six times during the twenty-four hours, and at each of these periods shows more or less variation in its physical characters. We have found that the urine which collects in the bladder during the night, and is discharged the first thing in the morning, is usually dense,

highly colored, of a strongly acid reaction, and a high specific gravity. That passed during the forenoon is pale, and of a low specific gravity, sometimes not more than 1018 or even 1015. It is at the same time neutral or slightly alkaline in reaction. Toward the middle of the day, its density and depth of color increase, and its acidity returns. All these properties become more strongly marked during the afternoon and evening, and toward night the urine is again deeply colored and strongly acid, and has a specific gravity of 1028 or 1030.

The following instances will serve to show the general characters of this variation:-

These variations do not always follow the perfectly regular course manifested in the above instances, since they are somewhat liable, as we have already mentioned, to temporary modification from accidental causes during the day; but their general tendency nearly always corresponds with that given above.

It is evident, therefore, that whenever we wish to test the specific gravity and acidity of the urine in cases of disease, it will not be sufficient to examine any single specimen taken at random; but all the different portions discharged during the day should be collected and examined together. Otherwise, we should incur the risk of regarding as a permanently morbid symptom what might be nothing more than a purely accidental and temporary variation.

The chemical constitution of the urine as it is discharged from the bladder, according to the analyses of Berzelius, Lehmann, Becquerel, and others, is as follows:

We need not repeat that the proportionate quantity of these different ingredients, as given above, is not absolute, but only approximative; and that they vary, from time to time, within certain physiological limits, like the ingredients of all other animal fluids.

The urea, creatine, creatinine, and urates have all been sufficiently described above. The mucus and coloring matter, unlike the other ingredients of the urine, belong to the class of organic substances proper. They are both present, as may be seen by the analysis quoted above, in very small quantity. The coloring matter, or urosacine, is in solution in a natural condition of the urine, but is apt to be entangled by any accidental deposits which may be thrown down, and more particularly by those consisting of the urates. These deposits, from being often strongly colored red or pink by the urosacine thus thrown down with them, are known under the name of "brick-dust" sediments.

The mucus of the urine comes from the lining membrane of the urinary bladder. When first discharged it is not visible, owing to its being uniformly disseminated through the urine by mechanical agitation; but if the fluid be allowed to remain at rest for some hours in a cylindrical glass vessel, the mucus collects at the bottom, and may then be seen as a light cottony cloud, interspersed often with minute semi-opaque points. It plays, as we shall hereafter see, a very important part in the subsequent fermentation and decomposition of the urine.

Biphosphate of soda exists in the urine by direct solution, since it is readily soluble in water. It is this salt which gives to the urine its acid reaction, as there is no free acid present in the recent condition. It is probably derived from the neutral phosphate of soda in the blood, which is decomposed by the uric acid at the time of its formation; producing, on the one hand, a urate of soda, and converting a part of the neutral phosphate of soda into the acid biphosphate.

The phosphates of lime and magnesia, or the "earthy phosphates," as they are called, exist in the urine by indirect solution. Though insoluble, or very nearly so, in pure water, they are held in solution in the urine by the acid phosphate of soda, above described. They are derived from the blood, in which they exist in considerable quantity. When the urine is alkaline, these phosphates are deposited as a light colored precipitate, and thus communicate a turbid appearance to the fluid. When the urine is neutral, they may still be held in solution, to some extent, by the chloride of sodium, which has the property of dissolving a small quantity of phosphate of lime.

The remaining ingredients, phosphates of soda and potass, sulphates and chlorides, are all derived from the blood, and are held directly in solution by the water of the urine.

The urine, constituted by the above ingredients, forms, as we have already described, a clear amber colored fluid, with a reaction for the most part distinctly acid, sometimes neutral, and occasionally slightly alkaline. In its healthy condition it is affected by chemical and physical reagents in the following manner.

Boiling the urine does not produce any visible change, provided its reaction be acid. If it be neutral or alkaline, and if, at the same time, it contain a larger quantity than usual of the earthy phosphates, it will become turbid on boiling; since these salts are less soluble at a high than at a low temperature.

The addition of nitric, or other mineral acid, produces at first only a slight darkening of the color, owing to the action of the acid upon the organic coloring matter of the urine. If the mixture, however, be allowed to stand for some time, the urates of soda, potass, &c., will be decomposed, and pure uric acid, which is very insoluble, will be deposited in a crystalline form upon the sides and bottom of the glass vessel. The crystals of uric acid have most frequently the form of transparent rhomboidal plates, or oval lamina with pointed extremities. They are usually tinged of a yellowish hue by the coloring matter of the urine which is entangled with them