EXCRETION 207 



it comes in contact with the acid urine. If the animal be killed 

 a short time after the administration of the indigo, the con- 

 torted portions of its tubules and the ascending limbs of the 

 loops of Henle are strongly coloured blue. An ammoniacal 

 solution of carmine may be used for a similar experiment ; but 

 the results are not nearly so sharply limited to the large-celled 

 portions of the tubules. Even the glomerulus is coloured red, 

 a fact which has been interpreted as showing that, although 

 the greater part of the carmine is excreted into the tubules, 

 some of it accompanies the water which exudes from the blood 

 through the glomerular tufts. 



The practical identity in structure of the kidney in birds and 

 reptiles and mammals would seem to have an important bearing 

 on this controversy. The urinary excretion of birds consists 

 almost exclusively of uric acid. As seen under the microscope, 

 it is a semisolid white deposit, made up of crystals, supposing 

 no special precautions have been taken to obtain it fresh. The 

 water, pigment, and salts which are essential elements of the 

 excretion of mammals are practically absent. Yet the kidney 

 of a bird presents the same arrangement of glomeruli and 

 tubules as the kidney of a mammal, although the glomeruli are 

 relatively smaller. Uric acid diffuses with great difficulty. 

 If it is, so to speak, washed through the glomeruli, and the 

 water which dissolved it reabs orbed by the tubules, an enor- 

 mous quantity of water must pass through the kidney in order 

 that it may carry the uric acid in its stream. If uric acid be 

 excreted by the epithelium of the tubules, it is difficult to 

 account for the presence of glomeruli, since no water leaves the 

 kidney. Crystals of uric acid are to be seen in a section of the 

 kidney, not only in the cells of the tubules, but also in the 

 glomeruli ; but it may well be that in both situations crystalliza- 

 tion has been induced during the preparation of the section. It 

 jars an histologist's conception of the constitution of a secreting 

 cell to contemplate the formation within its network of proto- 

 plasm, and the extrusion from it, of sharp-angled crystals. As 

 a matter of fact, it is not in its crystalline form that uric acid is 

 excreted by birds, but as quadri-urates i.e., salts containing 

 only one-fourth of their " normal " complement of base ; 

 crystalline spheres or amorphous deposit, not angular crystals. 

 These quadri-urates decompose very quickly, setting free 



