THE KIDNEYS AND SKIN. 487 



indeed, the absence of peptone (or of all but the merest traces 

 of it) from healthy human urine is one of the main reasons 

 for believing that peptone absorbed from the alimentary canal 

 is converted at once by the lymphoid tissues of the mucous 

 membrane into the proper proteids of the blood plasma. 

 When sugar appears in the urine either in disease or, as some- 

 times happens temporarily, in health, after a meal rich in 

 starchy substances we have from the results of experiment 

 on amphibia reason to believe that its path of excretion is 

 through the glomeruli. In hcvmoglobinuria, a condition in 

 which haemoglobin is found in solution in urine (not in blood- 

 corpuscles, for in that case it may have come from ruptured 

 vessels anywhere in the renal apparatus), the haemoglobin also 

 passes out through the Malpighian bodies: for when some 

 laky blood (Chap. IV) is injected into the vessels of an animal 

 and the secretion of urine at the same time made slow, col- 

 lections of haemoglobin may be found in the cavities of the 

 capsules. While, however, we have evidence that the epithe- 

 lium of the capsule has a certain selective power and is the 

 special seat of transmission of particular, especially abnormal, 

 urinary constituents, yet on the whole the glomeruli provide 

 a merely physical apparatus. Through them most of the 

 bulk of the urine passes out, and, flushing the more active 

 portions of the tubules on its course to the pelvis of the kid- 

 ney, picks up from them the more specific urinary con- 

 stituents. 



Urea is the most important and most abundant of the 

 characteristic ingredients of urine, and it has a very marked 

 influence on kidney activity, the injection of some of it into 

 blood causing a greatly increased secretion of urine, in which 

 the injected urea is quickly passed out. Judging from ex- 

 periments on amphibia, urea is not excreted or at any rate not 

 chiefly excreted by the glomeruli. On tying the renal arte- 

 ries of one of these animals urinary secretion ceases, there 

 being then no blood-pressure in the glomeruli to cause the 

 transudation of liquid; but if some urea be now injected 

 into the blood the ephithelial cells of other parts of the 

 tubules are stimulated to secrete, and urine rich in urea is 

 formed; but in these circumstances it cannot come from the 

 Malpighian bodies. It would seem then that urea is a special 

 stimulant to some cells of the tubules, and that an excess 

 of it in the blood can stir them up to its elimination along 



