FORMATION OF URINE 573 



development, pointed out that two distinct mechanisms 

 exist — 



1st. In the Malpighian bodies, an arrangement manifestly 

 suited to allow of filtration from the blood. 



2nd. In the tubules, a series of secreting structures. 



A. Malpighian Bodies. 



1. It has been shown by injecting acid fuchsin, which is 

 colourless in alkaline solution and red in acid solution, into the 

 blood-vessels that the urine formed in these bodies is alkaline 

 in reaction. It becomes acid as it passes down the convoluted 

 tubules, 



2, It is also known that these bodies are thrown out of 

 action by lowering the pressure in the renal arterioles and by 

 decreasing the flow of blood through the kidney. 



The amount of blood in the kidneys may be measured by 

 enclosing the organ in a closed vessel with rigid walls connected 

 with a piston recorder — an oncometer — so that changes in the 

 volume of the organ may be recorded, while the rate of flow may 

 be estimated by measuring the amount of blood coming from 

 the renal vein (p. 473). These two methods are frequently used 

 in combination. 



(a) Section of the splanchnic nerves to the kidney causes a 

 dilatation of the renal arterioles, an expansion of the kidney, and 

 an increased flow of urine. (6) Stimulation of these nerves has 

 the opposite effect. Sometimes stimulation with slow induction 

 shocks may cause a dilatation, but the action of these dilator fibres 

 is generally masked by that of the constrictors, (c) A fall in the 

 general arterial pressure, to about 50 mm. Hg in the dog, causes 

 a decreased flow of blood through the kidney and practically 

 stops the flow of urine, although the tubules, as will presently 

 be shown, still act. (d) Conversely, a stoppage of the formation 

 of urine may be brought about by raising the pressure in the 

 ureter to about 50 mm. Hg. 



3. In the frog the renal arteries supply the Malpighian 

 bodies, while portal veins, from the posterior end of the animal, 

 supply the convoluted tubules. Ligature of the renal arteries 

 stops the flow of urine ; but the flow may be again induced by 

 injecting urea and other substances. 



4. Even when this flow is induced dextrose, egg albumin 



