426 VETERINARY PHYSIOLOGY 



contains a higher proportion of these than does the blood, e.g. 

 the blood contains only about 0'03 per cent, of urea, but the 

 urine usually contains 2 per cent. The addition of solids in 

 the tubules is further proved by the following facts : 



(a) Uric acid crystals are frequently found in the cells of 

 the convoluted tubules of the kidney of birds. (6) Heidenhain, 

 by injecting a blue pigment sulph-indigotate of soda into 

 the circulation of the rabbit, demonstrated that the cells of 

 the convoluted tubules take it up and pass it into the urine. 

 In the normal rabbit the whole of the kidney and the urine 

 become blue. But, if the formation of urine in the Malpighian 

 bodies be stopped by cutting the spinal cord in the neck so as 

 to lower the blood pressure, then the blue pigment is found in 

 the cells of the convoluted tubules and of the ascending limb 

 of Henle's tubule, (c) When the Malpighian bodies of the 

 frog have been thrown out of action by ligaturing the renal 

 arteries, the injection of urea still causes a flow of urine and 

 the excretion of urea by the tubules. When the portal veins 

 which supply the tubules were ligatured on one side, it was 

 found that less urine was formed on the ligatured than on the 

 unligatured side. 



These last experiments show that the cells of the convoluted 

 tubules are capable of secreting water as well as solids, and 

 that they can do so is further demonstrated by the fact that, 

 if by cutting the spinal cord in the neck the formation of urine 

 in the Malpighian bodies of a dog is stopped, the administration 

 of caffeine and of some other substances causes an increased 

 flow of urine, although the blood pressure in the kidneys is not 

 raised. This is taken advantage of in cases of heart disease, 

 when the secretion of urine is almost arrested from low arterial 

 pressure, and when dropsy is rapidly advancing. Until the 

 heart is toned up, the kidneys may be stimulated to get rid 

 of water by means of such diuretics as caffeine. 



It is quite possible that under certain conditions the cells of 

 the renal tubules absorb water and possibly the other con- 

 stituents of the urine, but the weight of the evidence we at 

 present possess is against the idea that they exercise the function 

 to any great extent. 



