546 THE URINARY APPARATUS [CH. XXXVI. 



or stimulating nerves corresponds to blood-pressure. A rise of blood- 

 pressure in the renal artery is produced by constriction of the renal 

 arterioles; this is accompanied by a fall of pressure in the renal 

 capillaries, and a shrinkage of the kidney. Increase in the volume 

 of the kidney is produced by the opposite circumstances. 



The accompanying tracing (fig. 424) shows that in a kidney curve 

 one gets a rise of volume due to each heart-beat, and larger waves 

 which accompany respiration. In many cases larger sweeping waves 

 (Traube-Hering curves) are often shown as well. If a kidney curve 

 is compared with a tracing of arterial pressure, it will be seen that 

 the rise of arterial pressure coincides with a fall of the oncograph 

 lever due to constriction of the renal vessels. 



Diuretics are drugs which produce an increased flow of urine ; 

 they act in various ways, some by increasing the general blood- 

 pressure, others by acting locally upon the kidney (increasing its 

 volume as measured by the oncometer) ; under this latter head are 

 doubtless to be included some also which act on the renal epithelium 

 rather than on the blood-vessels. 



Activity of the Renal Epithelium. 



The epithelium of the convoluted tubules has a structure which 

 suggests from its resemblance to other forms of secreting epitheliums, 

 that its function here also is secreting. This is confirmed by the 

 manner in which the blood-vessels break up into capillaries around 

 these tubules ; and is further confirmed by experiments. 



Heidenhain showed that if a substance called sodium sulphindigo- 

 tate, which ordinarily produces blue urine, is injected into the blood 

 (after section of the medulla oblongata, which causes lowering of the 

 blood-pressure in the renal glomeruli), when the kidney is examined, 

 the cells of the convoluted tubules and of these alone are stained 

 with the substance, which is also found in the lumen of the tubules. 

 This shows that the pigment is eliminated by the cells of the con- 

 voluted tubules, and that when by diminishing the blood-pressure, 

 the filtration of urine is stopped, the pigment remains in the 

 convoluted tubes, and is not, as it would be under ordinary circum- 

 stances, swept away from them by the flushing of them by the 

 watery part of the urine derived from the glomeruli. It therefore is 

 probable that the cells, if they excrete the pigment, excrete urea and 

 other substances also. 



But the proof is not absolute, for the pigment is a foreign 

 substance. Urea is a very difficult substance to trace in this way 

 because it does not leave any coloured trail behind it. In birds the 

 place of urea is taken by uric acid, and the urates can be actually 

 traced, because they are deposited as crystals, and can be seen in the 



