1 Ribbert, Virchow's Archiv, July 1883, p. 189.

2 Adami, Journ. of Phys., vol. vi. 1885.

Urea, on the contrary, is not only excreted by the tubules after ligature of the renal artery, but carries with it, in the process of secretion, from the venous plexus, a considerable quantity of water, so that the bladder becomes partially filled.

Fig. 148.  Diagram of the circulation in the kidney of the newt. Modified from Nussbaum.

Fig. 148. -Diagram of the circulation in the kidney of the newt. Modified from Nussbaum.

The excretion of water, therefore, takes place in a double manner : it passes out through the glomeruli when the renal arteries are free, and it passes out from the venous plexus along with urea, even although the renal arteries are tied.

Fig. 149.   Diagrammatic sketch of the blood vessels in a mammalian kidney. 0 is an artery ascending into the cortical substance of the kidney; p is a branch from it which divides into two branches, q and P.

Fig. 149. - Diagrammatic sketch of the blood-vessels in a mammalian kidney. 0 is an artery ascending into the cortical substance of the kidney; p is a branch from it which divides into two branches, q and P. q breaks up at once into a number of twigs. P is the afferent artery to a glomerulus, S, of the lowest row. t is the efferent vessel of the glomerulus. It divides into two branches, one of which, u, ascends towards the cortex, whilst the other, v, descends towards the medulla. (From Schweigger-Seidel, Die Nieren, Halle, 1865.)

In the kidneys of the higher animals (Fig. 149) and of man the glomeruli and the tubules do not receive blood from two entirely different sources; but there is an arrangement somewhat similar, for the plexus surrounding the tubules does not receive blood only from the efferent vessels of the Malpighian corpuscles, but gets it also directly from the renal arteries. There are three channels by which the blood may pass from the renal arteries into the venous plexus without going through the glomeruli. The first is the inosculation which takes place between the terminal twigs of the renal artery and the venous plexus on the surface of the kidney directly under the capsule.1 The second channel is formed by small branches given off directly by the interlobular arteries or by the afferent arteries before they reach the glomeruli. The former of these may be regarded as corresponding to the artery which passes directly to the plexus in the newt, and the latter to the branch connecting it with the afferent artery (Fig. 148). These arterial twigs are found not only near the surface of the kidney, but also in the deeper layers of the cortical substance.2 The third and most important channel is afforded by the arteriae rectae, which spring from the branches of the renal artery at the boundary between the cortical and medullary substance and pass into the medulla, where they form a plexus with elongated meshes surrounding Henle's loops and the collecting tubules. Near their origin the arteriae rectae

Fig. 150.   Diagram of the tubules and vascular supply of the kidney. On the left is a tubule alone; in the middle is a tubule along with the blood vessels; on the right are blood vessels only.

Fig. 150. - Diagram of the tubules and vascular supply of the kidney. On the left is a tubule alone; in the middle is a tubule along with the blood-vessels; on the right are blood-vessels only.

1 Ludwig, Handworterbuch d. Physiol., v. R. Wagner, Bd. 2.

2 Schweigger-Seidel, Die Nieren, p. 67; Heidenhain, Hermann's Handbuch d. Physiologic, vol. v., Th. 1, p. 293.

inosculate with the venous plexus surrounding the convoluted tubules (Fig. 150).

Through these three channels it is possible for blood to reach the secreting structures of the kidney and there get rid of urea, salts, etc, without losing water by its passage through the glomeruli. On the other hand, if these vessels contract, while the size of the renal artery and the pressure of the blood within it remain unaltered, more blood will be forced into the Mal-pighian corpuscles, and thus the quantity of water excreted will be increased. At the same time the contraction of the arteriae rectae will probably diminish absorption from the tubules, and thus the quantity of water excreted will be increased in a twofold manner.

Circumstances Modifying the Secretion of Urine. - The experiments of Ludwig and his pupils have shown that the amount of urine secreted depends very closely upon the pressure of blood in the Malpighian corpuscles, or, to put it more exactly, on the difference of pressure between the blood in these corpuscles and the pressure within the tubules. For if the ureter be tied so that the pressure of urine in the tubules is increased, the secretion is greatly diminished, and even arrested, even though the pressure of blood in the renal artery be high.

A somewhat similar effect to that of ligature of the ureter is produced by ligature of the renal vein, for the blood accumulating in the venous plexus surrounding the tubules compresses them so as to prevent the flow of urine through them. A similar condition may occur from cardiac or pulmonary disease obstructing the venous circulation.

But unless under exceptional circumstances which alter the pressure within the tubules, such as compression of the tubules by congestion of the venous plexus, as in cardiac disease, impaction of a calculus in the ureter, or pressure on the ureters by dropsical accumulations or tumours, the rapidity of the secretion of urine depends on two factors: (1) arterial pressure in the glomeruli; and (2) the composition of the blood.