. THE KIDNEYS AND SKIN. 439 



A blue substance known as sodium sulphindogate after in- 

 jection, in solution, into a vein of an animal is excreted in 

 the bile and urine. If the animal be killed during the 

 excretion no traces of this body can under normal circum- 

 stances be detected in any special part of the kidney; it is 

 in fact washed away by the urine as fast as the cells pick it 

 up and pass it into the tubuli. But if the blood-pressure of 

 the animal be made so low (as by cutting the main vaso- 

 constrictor nerves) as to bring the renal secretion to a stand, 

 and the animal be killed some time after injection of the 

 indigotate, the glomeruli and most of the tubules are found 

 free of the blue, which lies only in the contorted portions, 

 just where the cells which gathered it from the circulating 

 liquid had passed it out. 



Though the renal epithelium does not make urea it 

 has constructive powers as regards some other urinary constit- 

 uents. As already stated, benzoic acid taken with the food 

 leaves the Body as hippuric acid, having been combined 

 with glycin. If blood containing benzoic acid and glycin be 

 artificially circulated through a perfectly fresh still living 

 kidney, the renal vein blood will contain hippuric acid. 

 Even if no glycin be provided in the blood injected through 

 the renal artery the returning blood will still yield hippuric 

 acid. So living kidney cells can not only perform the 

 synthesis, with dehydration, necessary to form hippuric acid, 

 but can also form and supply the required glycin. The 

 process is closely dependent on the vitality of the cells; the 

 experiment fails if the organ be not perfectly fresh and unin- 

 jured, and if the blood supplied be not properly arterialized. 



The Influence of Renal Blood-flow on the Amount of 

 Urine Secreted. From the structure of the glomeruli and 

 the fact that most of the water of the urine is derived 

 from them it is a priori probable that anything tending to 

 increase the pressure of blood in them will increase the bulk 

 of urine secreted, and anything diminishing that pressure 

 decrease the urine. This is confirmed by experiment. The 

 kidney is supplied with both vaso-constrictor and vaso-dilator 

 nerves which reach it mainly through the solar plexus, 

 though both sets come ultimately from the spinal cord. 

 When the spinal cord is cut in the neck region of a dog the 

 kidney vessels as well as those of the rest of its body dilate 

 and blood-pressure everywhere is very low. Under these 



