702 SECRETION OF UREA. [BOOK n. 



fections in the glomerular epithelium. But even under unhealthy 

 conditions that epithelium still governs to a certain extent the 

 passage of material ; for the proteids of the blood-plasma do not 

 pass through bodily or in a proportion which corresponds either 

 to the relative proportion in which they exist in the plasma or to 

 the relative ease (or difficulty) with which they pass through 

 membranes. Though the " albumin " of albuminous urine fre- 

 quently consists of both serum-albumin and globulin, these do 

 not necessarily occur in the same proportion as in blood ; they 

 vary in urine much more than they do in blood ; and indeed the 

 one or the other may be absent ; moreover fibrin factors are very 

 rarely found. 



Hsemoglobinuria, or the presence of hemoglobin in urine, may 

 be brought about by injecting into the blood vessels 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 hemoglobin 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 glomeruli, 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. 



418. The work of the epithelium of the tubules. As we have 

 said the structural features of the epithelium 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 sub- 

 stances 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 different. In the 

 first place urea is always present in blood ; in the dog for instance 

 it is found to an extent varying from "035 p.c. in hunger to 

 153 p.c. after heavy feeding. In the second place, if the kidneys 

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



