The kidney has a twofold office. It has (1) to regulate the amount of water in the body under various conditions; (2) to remove the products of tissue-waste. These products must be removed in a state of solution from the part of the kidney where they are excreted, and yet sometimes provision must be made for the water, by which they are washed out, being retained in the body. The urine in mammals and amphibia is liquid; in birds and reptiles it is semi-fluid or solid, yet the solid constituents are removed in solution from the urinary tubules, and the water in which they are dissolved is afterwards absorbed. We may say then that the kidney has not only a twofold, but a threefold action: - 1st, the excretion of waste-products; 2ndly, a provision for the removal of excessive water; and 3rdly, an arrangement for the retention of water in the body, by its re-absorption after it has washed out the waste-products. On looking at the kidney we find three structures which seem to be connected with these three functions, viz.: (1) convoluted tubules with epithelial cells, which in all probability are the chief structures for excreting waste-products; (2) the Malpighian corpuscles for excreting water along with some solids, and (3) usually one or more constrictions in the tubule which may serve the purpose of preventing too rapid exit of the water, and thus allow time for its re-absorption in cases where its retention is desirable, as for example on a hot day and when the supply of drinking-water is very limited.

The process of secretion in the kidney was regarded by Bowman as consisting of the filtration of water from the vessels of the glomeruli into the tubules, and the excretion of waste-products by the epithelium lining the tubules. Ludwig, however, came to look upon it rather as a process of filtration and re-absorption; a dilute solution of urea and salt being, according to him, poured out from the Malpighian corpuscles and gradually concentrated by the absorption of water in its passage along the tubules. This theory had so many facts in its favour that it wasfor a good while exclusively adopted, but latterly Heidenhain, in an admirable series of experiments, has shown that such substances as indigo are certainly excreted by the epithelium of the tubules. At the same time Hufner has shown, by a comparison of the structure of the kidney in fishes, frogs, tortoises, birds, and mammals, that the form of the tubules closely agrees with that required for the re-absorption of water in each case. Fishes have a low blood-pressure, and so the resistance in the kidney requires to be small in order to allow of the secretion of urine. Living as they do in water, they do not require any apparatus for its retention in the body. In them therefore the tubule is short and wide, and destitute of any constriction which would retard the outflow of the fluid. In frogs there must be ample provision for the retention of water in the body, as evaporation takes place freely from their skin. In them we find, as we might expect, that the tubule, and especially the contracted part of it, is very long. In tortoises no evaporation from the skin can take place, and in them the contracted part of the tubule is short. This renders it probable that, while the ideas advanced by Bowman and supported by Heidenhain are in the main true, the re-absorption of water on which Ludwig lays so much stress is also an important factor in the secretion of urine under different circumstances.

Fig. 147.   Diagram showing the form of the urinary tubules in different classes of animals, after Hufner. 1. Pish. 2. Frog. 3. Tortoise. 4. Bird. 5. Mammal.

Fig. 147. - Diagram showing the form of the urinary tubules in different classes of animals, after Hufner. 1. Pish. 2. Frog. 3. Tortoise. 4. Bird. 5. Mammal. The letters have the same signification in each. a. Capsule of the glomerulus. b. Convoluted tubule. c. Loop. d. Collecting tubule. u in 2 indicates the transverse section of the ureter.

But it is not only rendered probable by the facts of comparative anatomy; it appears to be proved by direct experiment. Ribbert1 has extirpated the medullary substance of the kidney in the rabbit while leaving the cortical substance. He has thus succeeded in collecting the urine as it is excreted by the Mal-pighian corpuscles before it has passed through Henle's loops, and has found that the urine secreted by the cortical substance alone is much more watery than that which is secreted by the entire kidney - a fact which appears conclusively to prove that water is actually re-absorbed, and the urine rendered more concentrated, during its passage through the tubules of the medullary substance.

In the frog and triton the arrangement of the kidney is such as to allow of a much more complete investigation of the different factors in secretion, than in mammals, because in amphibia, the glomeruli which separate the water and the tubules which excrete the solids, receive their blood-supply to a great extent independently. The glomeruli are supplied by branches of the renal artery. The tubules are supplied by a vein which proceeds from the posterior extremities and, entering the kidney, breaks up into a capillary plexus bearing a somewhat similar relation to the renal tubules as that which the portal vein does to the lobules of the liver. It is therefore called the portal vein of the kidney.

The arterial circulation in the glomeruli and the venous portal circulation round the tubules are not entirely distinct, for the efferent arteries of the glomeruli unite with the portal capillaries, and, moreover, arterial twigs also pass directly from the renal artery into the capillary venous plexus (vide Fig. 148). The two systems are so far distinct that Nussbaum has been able to ascertain with considerable exactitude the part played by each in secretion, although Adami2 has shown that the communication is freer than Nussbaum supposed. By ligaturing the renal artery Nussbaum destroyed the functional activity of the glomeruli, and by ligaturing the portal vein of the kidney he destroyed that of the tubules. By injecting a substance into the circulation after ligature either of the artery or the vein, and observing whether it is excreted or not, he determines whether it is excreted by the glomeruli or by the tubules. In this way he finds that sugar, peptones, and albumen pass out through the glomeruli exclusively, for they are not excreted when the renal arteries are tied. Albumen, however, only passes out through the glomeruli when an abnormal change has already occurred in the vascular wall; as, for example, after the circulation has been arrested for a while by ligature of the renal artery. Indigo-carmine, when injected after ligature of the renal arteries, passes into the epithelium of the tubules, but it does not give rise to any secretion of water, so that the bladder is found empty.