EXCRETION 405 



ments, although not perhaps quite conclusive, have made it 

 probable that in the frog urea is actually separated by the 

 epithelium of the tubules. 



The experiments of Heidenhain and Nussbaum deserve more 

 detailed description. The former injected indigo-carmine 

 into the blood of rabbits and after a variable time killed 

 them, cut out the kidneys, 

 and flushed them with alcohol. 

 His results were as follows : 

 (i) When the spinal cord was 

 cut before the injection in order 

 to reduce the blood-pressure, 

 the blue granules were found in 

 the ' rodded ' epithelium of the 

 convoluted tubules and the 

 ascending limb of Henle's loop, 

 and in the lumen of the tubules, AFTER INJECTION INTO BLOOD. 



but nowhere else. The renal The cortex between a and b and 



between c and d was cauterized before 

 Cortex was Coloured blue, the injection. In the black wedge- 



(2) When the spinal cord was 

 not cut, the pigment was found 

 in the medulla and pelvis of the 

 kidney, as well as in the cortex, but always in the lumen 

 of the tubules, and not in the epithelium, except in the 

 situations mentioned. (3) If a portion of the cortex of the 

 kidney had been cauterized with nitrate of silver before 

 injection of the pigment, the spinal cord being left intact, 

 a wedge of the renal substance, corresponding to this area, 

 remained coloured only in the cortex, although the rest 

 was blue in the medulla also. The rodded epithelium was 

 filled with blue granules as before (Fig. 127). 



(i) shows that the epithelium is capable of excreting 

 some substances at least. (2) appears to show that when 

 the blood-pressure is normal water is poured out from some 

 part of the tubule, and washes the pigment separated by the 

 ' rodded ' epithelium down towards the papillae. (3) suggests 

 that it is through the glomeruli that most of the water 

 passes. For cauterization has not destroyed the power of 

 the epithelium to excrete pigment, and therefore, presumably, 



