out of the convoluted tubes by the stream of fluid filtered from the blood at the glomerulus.

The following facts may also be adduced in further support of the view that the glandular epithelium has a considerable share in the removal of the more important solid constituents of the urine.

The epithelium in the tubules of the kidney of birds is found impregnated with acid urates, which form the chief solid constituents of the urine of birds.

The amount of liquid passing out at the kidneys is in direct proportion to the blood pressure, whereas the excretion of the specific constituents of urine is independent of the pressure, but is related to the amount existing in the blood, and the condition of the epithelium. This is shown by the increased elimination of urea when that substance is artificially introduced into the circulation, even after the flow of the fluid has been checked by section of the spinal cord.

Another view has been put forward, which, with some modification, appears plausible, or at least worthy of mention. Paying attention to the fact that where vascular filtration-i. e., the passage of liquid under pressure through the capillary wall-occurs elsewhere in the body it is not only water and salts, but plasma that passes out of the vessels into the interstices of the tissues, we may then assume that the fluid part of the blood, as such, and not merely its watery part, escapes at the glomerulus. That is to say, the solid ingredients of the urine in a diluted form, plus serum-albumin, pass into the tubules. But on its way down the long and circuitous route through the tubules the albumin with much water is reabsorbed by the capillaries of the convoluted tubes. The first step in this case is a mechanical filtration ; the second is a vital process of reabsorption of a solution of serum-albumin carried on by the gland cells in the tubules, aided by the low pressure in the peri-tubular capillary plexus. This view seems supported by pathological experience, which teaches that the removal of the epithelium of the tubes (the glomeruli remaining perfect), is followed by the appearance of albumin in the urine, and cysts formed by the destruction of the

epithelium and occlusion of the tubules commonly contain a fluid somewhat like plasma.

Doubtless much remains to be found out as to the exact method of secretion of the urine, and possibly future research may show us that all the views here enumerated have some truth in them. That a filtration, not mere osmosis, takes place, seems probable from the special vascular mechanism of the glomerules. Why simply water and salts without albumin should pass through the capillaries of the glomerulus and not through any other capillaries, is not sufficiently explained to make it sure that such a filtration really occurs. That the glandular epithelium does take an active part in the elimination of the urea is rendered almost indisputable from the researches of Heidenhain. And yet there remain other parts, e. g., the loops of Henle, which are invariably found in the kidney, and have a special vascular mechanism, to which none of the foregoing theories assign any special or peculiar function.

From the foregoing evidence we may fairly suppose that most of the urea, and possibly some other solid constituents of the urine, are selected from the blood by the epithelial cells of the convoluted tubules, that the fluid part of the blood escapes at the glomerulus, and flows along the varied and circuitous route of the tubules, carrying with it the matters poured into the tubes by the cells, and that in some part of the tubules the dilute filtrate loses much of its water and all its albumin.

CHEMICAL COMPOSITION OF URINE.

The percentage of the solid and liquid materials in urine varies as the secretion alters in strength, but on an average it may be said to contain about 4 per cent. of solids and 96 per

cent. water.

The following are the more important solid matters:

Urea is the most important, and at the same time most abundant solid constituent, commonly forming about 2 per cent. of the urine. It is regarded as the chief end-product of the oxidation of the nitrogenous matter in the body, so that the amount excreted per diem gives us the best estimate of the amount of

It

chemical change taking place in the nutrition of the tissues. is readily soluble in alcohol and water, but insoluble in ether. It forms acicular crystals with a silky lustre. From a chemical point of view it may be regarded as the monamide of carbamic (NH2 acid, with the formula CO {NH2 It is isomeric with ammo

CN

nium cyanate NHO, from which it was first prepared arti

ficially.

On exposure to the air bacteria develop in the urine, and, acting as a ferment, change the urea into ammonium carbonate, two molecules of water being at the same time taken up thus:CO(NH), 2H,O= (NH),CO3.

:-

This gives rise to a change in the reaction of the urine, which, after a time, becomes increasingly alkaline, and the change is commonly spoken of as the alkaline fermentation of the urine. This change is extremely slow in solutions of pure urea, which do not support bacterial life.

With nitric and oxalic acids, urea forms sparingly soluble salts -a fact made use of in its preparation from urine.

The amount of urea eliminated in the 24 hours is about 500 grains (35 grammes). The amount varies (1) in some degree with the amount of urine secreted; an increase in the amount of water being accompanied by a slight increase in the urea eliminated. Some materials, such as common salt, increase the water, and thereby also increase the urea. (2) The character and quantity of the diet influences most remarkably the quantity of urea given off, the amount increasing in direct proportion to the quantity of proteid consumed. Fasting causes a rapid fall in the amount of urea; even in the later days of starvation it continues to fall, but very slowly. (3) The amount differs with age, being relatively greater in childhood than in the adult (about half as much again in proportion to the body weight). (4) Many diseases have a marked influence on the amount of urea. In most febrile affections it increases with the intensity of the fever, while in disease of the liver it often notably decreases. In diabetes, if the consumption of food be very great, the daily

excretion of urea may reach nearly 4 oz. (100 grammes) or three times as much as normal.

evapo

Preparation. To obtain urea from human urine it is rated to one-sixth of its bulk, an excess of nitric acid is added, and it is left to stand in a cool place. Impure nitrate of urea separates from the fluid as a yellow crystallized precipitate. This sparingly soluble salt is caught on a filter, dried, dissolved in boiling water, mixed with animal charcoal to remove the coloring matter, and filtered while hot; when the filtrate cools, colorless crystals of nitrate of urea are deposited. The precipitate is dissolved in boiling water, and barium carbonate added as long as effervescence takes place, barium nitrate and urea being produced. This is evaporated to dryness, and the urea extracted with absolute alcohol, which, on evaporation, leaves crystals of pure urea.

Estimation.-Urea can be estimated volumetrically by the method of Liebig, which depends on the power of mercuric nitrate to give a precipitate with it. The sulphates and phosphates must be first removed by the addition of 40 cc. of a mixture of volume saturated barium nitrate and 2 volumes saturated solution of caustic baryta, to 40 cc. of urine. This is filtered, and from the filtrate an amount corresponding to 10 cc. urine is taken. Into this known volume of urine a standard solution of mercuric nitrate (of which 1 cc. corresponds to 1 centigramme of urea) is dropped until a sample drop of the liquid, mingled on a watch glass with a drop of concentrated sodium carbonate solution, gives a yellow color, which indicates that some free mercuric nitrate is present. For every cubic centimetre of the standard mercuric solution used, there is one centigramme of urea in the sample of urine; a reduction of 2 cc. should be made from the mercuric solution used in the experiment, on account of the chlorides, which are present in tolerably

constant amount.

Another simple and more accurate method consists in mixing known quantities of urine and sodium hypobromite (NaBrO) with excess of caustic soda. The urea is decomposed in the presence of this salt, and free nitrogen evolved

CON,H,+3(NaBrO) + 2(NaOH) = 3NaBr + Na,CO, + 3H2O + 2N.

The quantity of urea may be determined by ascertaining the volume of nitrogen, which can be measured directly in a graduated tube. 37.5 cc. of N represents 0.1 gramme of urea at ordinary temperature and pressure.

Uric acid, of which the formula is C,H,N,O, or C1H2O2 (NH.CN), is only present in extremely small quantities in the normal urine of mammalia, but in birds, reptiles and insects it forms the chief ingredient of the renal secretion. It is sparingly soluble in water, and insoluble in alcohol and ether. However, in solutions of the neutral phosphates and carbonates of the alkalies it combines with some of the base, so as to form acid salts, and at the same time converts the neutral into acid phosphates, to which, as has been already stated, the urine owes its acid reaction. These salts are more soluble in warm than in cold water, and hence generally fall as a sediment when the urine cools. Uric acid is readily converted into urea by oxidation, and is probably one of the steps in the formation of urea generally occurring in the body during the gradual oxidation of the proteid bodies.

test.

The presence of uric acid may be recognized by the murexide The substance to be tested is gently heated in a flat capsule with some nitric acid. A decomposition occurs, N and CO, going off, urea and alloxan remaining as a layer of yellow fluid. If this be cautiously evaporated, and a drop of ammonia added, a striking purple red color is produced, which the addition of potash turns violet.

The amount of uric acid normally follows pretty closely the variations in urea, but is usually only about 8 grains (.5 gramme) per diem. In certain diseases the quantity may be much inFor the quantitative estimation, which is seldom decided by the practitioner, the student must consult the text-books of physiological chemistry.

Kreatinin (C,H,N,O) is always present in urine, probably being formed from kreatin by the loss of one molecule of water. About 15 grains (1 gramme) is excreted per diem.

Xanthin (CH,N,O) also occurs in urine, but in extremely small quantities.