406 HUMAN PHYSIOLOGY 



It may also be demonstrated by taking advantage of the 

 fact that in the frog the Malpighian bodies are chiefly, if 

 not entirely, supplied from the renal artery, and the tubules 

 from a portal vein. When sugar or commercial peptone is 

 injected into the circulation, it appears in the urine, but 

 if the renal arteries are first ligatured, it does not appear, 

 proving that it is by the gloineruli that it is passed out. 



II. Secretion in the Tubules. The alkaline urine formed in 

 the Malpighian bodies undergoes changes as it passes along 

 the tubules. It becomes acid and the various solids must 

 be increased, for urine contains a higher proportion of these 

 than does the blood. The blood contains only about 0'03 per 

 cent, of urea, but the urine usually contains 2 per cent. It 

 has been suggested that this is due to absorption of water 

 from the tubules, by which the urine formed in the Mal- 

 pighian bodies become more concentrated. But there is no 

 evidence that this takes place. On the other hand, the fol- 

 lowing facts seem to show that solids are added to the urine 

 by a process of secretion from the cells of the tubules. 



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

 the convoluted tubules of the kidney of birds. (6) Heiden- 

 hain, 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 is stopped by cutting the spinal cord in 

 the neck so as to lower the blood pressure, then the blue pig- 

 ment is found in the cells of the convoluted tubules and of 

 the ascending limb of Henle's tubule, (c) When the Mal- 

 pighian 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 that substance by the 

 tubules. 



This last experiment seems to show that the cells of the 

 convoluted tubules are capable of secreting water, 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 ad- 

 ministration of caffeine and of some other substances causes 



