538 SECRETION OF UREA. [BOOK 11. 



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 " albu- 

 min " of albuminous urine frequently 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 haemoglobin 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 haemoglobin passes through the glom- 

 eruli ; 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. 



338. 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 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 extir- 

 pated there is no accumulation in the blood of either bile acids 

 or bile pigment. With regard to the kidney in relation to the 

 most important constituent of urine, namely urea, the case is 

 different. 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 



