422 THE ELIMINATION OF WASTE PRODUCTS. 



Hsernoglobinuria, or the presence of haemoglobin in urine, may be brought 

 about by injecting into the bloodvessels laky blood, or some substance such 

 as pyrogallic acid, which will " break up" the corpuscles of the blood. Now, 

 in such cases there is evidence that the haemoglobin passes through the 

 glomeruli ; minute disc-like masses of haemoglobin, the so-called " menisci," 

 are, by appropriate methods of preparation, found in situ in the capsules. 

 Such a passage is very far removed from being a process of diffusion. 



We may conclude, then, that the passage of material through the glom- 

 eruli, like the transudation of lymph, and even to a more marked extent, is 

 a complex affair in which the ordinary physical processes of diffusion and 

 filtration may play their part, but are not masters of the situation. 



354. The work of the epithelium of the tubules. As we have said, the 

 structural features of the epithelial cells of the tubules seem to justify the 

 conclusion that they exercise a secretory,activity comparable with that of a 

 salivary or a gastric gland. But their work is in many ways peculiar. In 

 the case of the salivary, gastric, and pancreatic glands there can be no doubt 

 that the specific constituents of the several secretions, mucin, pepsin, trypsin, 

 and the like, are manufactured in the alveolar cells out of antecedents of 

 some nature or other. The evidence, as we have seen, is all against the 

 view that these glands merely withdraw, secrete in the old sense of the word, 

 from the blood these substances preexisting in the blood. When the salivary 

 glands are extirpated, or the pancreas or the stomach removed, there is no 

 accumulation in the blood of the specific constituents of the corresponding 

 secretions. So also when the liver is extirpated there is no accumulation in 

 the blood of either bile acids or bile pigment. With regard to the kidney 

 and the most important constituent of urine, namely, urea, the case is differ- 

 ent. If the kidneys in a mammal be extirpated, or if the kidneys by disease 

 or by ligature of the ureters be so damaged as to be unable to carry on their 

 work, an accumulation takes place in blood, not as was once thought, of some 

 antecedent of urea, such as kreatin, but of urea itself. In the case of birds 

 and reptiles which excrete not urea, but chiefly uric acid, the accumulation 

 is one of uric acid. Obviously in secreting urea the work of the epithelium 

 of the tubules is largely, if not exclusively, confined to simply picking the 

 urea out of the blood and pushing it, so to speak, into the lumina of the 

 tubules. We might, perhaps, say exclusively, for there is no evidence that 

 any urea at all is actually manufactured in the kidney. 



But even this mere picking up the urea is after all not a simple process ; 

 the epithelial cell of the tubule is not a mere passive sieve of peculiar struc- 

 ture, especially adapted to strain off the urea from the blood. As we have 

 already seen, when urea or uric acid is injected into the blood the result is 

 not a mere increase in the proportions of urea (or uric acid) present in the 

 urine which is being secreted. The injection leads to an increased flow of 

 urine, the whole activity of the cell is stirred up, and other constituents, not 

 at the moment like the'urea existing in excess in the blood, are discharged 

 into the lumina of the tubules together with the urea, 



How the urea, which is in this peculiar manner taken out of the blood, 

 comes to make its appearance in the blood is a problem in which the kidney 

 is not concerned, and with which we shall deal in treating of the metabolic 

 events of the body generally. 



355. In the case of some other constituents of the urine we have evi- 

 dence that the cells do something more than simply pick the constituent out 

 of the blood. Hippuric acid, as we have seen, occurs in small quantity in 

 the urine of man, and in larger amount* in the urine of herbivora. Now, 

 hippuric acid may be formed by the combination, with dehydration, of ben- 

 zoic acid and glycin (C 7 H 6 O 2 +C 2 H 5 NO 2 ~-H,O^C 9 H 9 NO 3 ) ; and benzoic 



