FORMATION OF URINE 577 



ing 28,500 cc. ! The process, on the face of it, seems somewhat 

 wasteful. 



But such evidence as has been procured must be examined 

 on its merits, and the reabsorption theory cannot be rejected 

 simply because it seems improbable. 



1. Uric acid crystals are frequently found in the cells of the 

 convoluted tubules of the kidney of birds. Further, nric acid is 

 very soluble in piperazine, and when injected in solution of this 

 substance into the veins of a mammal, the uric acid appears in 

 the tubules and in the cells of the convoluted tubules, but not 

 in the gflomeruli or in the medulla. 



2. Heideuhain, by injecting into the circulation of the rabbit 

 a blue pigment — sulph-indigotate of soda — found that the cells 

 of the convoluted tubules take it up and seem to pass it into 

 the urine. In the normal rabbit the wliole of the kidney and 

 the urine became 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, since it is not washed out 

 of these. The fact that later investigators have found after 

 injection of aniline blue and Congo red, that these pigments 

 appear first in the part of the cells next the lumen of the tubules 

 seems of small significance. They might well accumulate there 

 before being excreted. 



The subcutaneous injection into rats of pyrrol blue leads to the 

 appearance of the pigment in the cells of the convoluted tubules 

 but not in the Malpighian bodies nor in the collecting tubules. 



3. 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, are ligatured on one side, it is found that less 

 urine is formed on the ligatured than on the uniigatured 

 side. 



4. If the formation of urine in the Malpighian bodies of a 

 dog be stopped by cutting the spinal cord in the neck, the 

 administration of certain substances such as of caffeine, 

 or NaoSO^, causes an increased flow of urine, although the blood 

 pressure in the kidneys is not raised. In this diuresis the 



37 



