CHEMISTRY OF URINE 489 



of strong nitric acid in the cold. Proteoses (albumoses) are also occa- 

 sionally present, e.g., in the disease called ' osteomalacia ' and in con- 

 ditions associated with the formation and especially with the decom- 

 position of pus. They may be recognized by the tests given in the 

 Practical Exercises (p. 525). It is doubtful whether the presence of 

 true peptone has as yet been satisfactorily made out. 



The presence of bile-salts may be shown by Hay's test or Petten- 

 kofer's test (p. 46-2). 



The pigments of blood and bile may be detected by the characters 

 described in treating of these substances; the spectrum of oxyhaemo- 

 globin, ormethaemoglobin, or any of the other derivatives of haemoglobin, 

 with the formation of haemin crystals, would afford proof of the presence 

 of the former, and Gmelin's test of the latter. The red blood-corpuscles, 

 seen with the microscope, are the most decisive evidence of the presence of 

 blood, as leucocytes in abundance are of the presence of pus. It should 

 be remembered that pus in the urine of women has sometimes no signifi- 

 cance except as showing that there has been an admixture of leucorrheal 

 discharge from the vagina. (See Practical Exercises, pp. 74, 531.) 



SECTION II. THE SECRETION OF THE URINE. 



We have now to consider the mechanism by which the urine is 

 formed in the kidney from the materials brought to it by the blood. 

 And here the same questions arise as have already been discussed 

 in the case of the salivary and other digestive glands: (i) Are the 

 urinary constituents, or any of them, present as such in the blood ? 

 (2) If they do exist in the blood, can they be shown to be separated 

 from it by processes mainly physical or mainly ' vital ' in other 

 words, by ordinary nitration, diffusion and osmosis, or by the selec- 

 tive action of living cells ? In the case of the digestive juices it 

 has been seen that some constituents are already present in the 

 blood, but that physical laws alone, so far as we at present under- 

 stand them, cannot explain the proportions in which they occur in 

 the secretions, or the conditions under which they are separated; 

 while other constituents and these the more specific and important 

 are manufactured in the gland-cells. 



In the kidneys the conditions seem at first sight favourable to 

 physical separation, as opposed to physiological secretion. Urine 

 has been described as essentially a solution of urea and salts, and 

 both are ready formed in the blood. The arrangement of the blood- 

 vessels, too, suggests an apparatus for filtering under pressure. 



Bloodvessels and Secreting Tubules of Kidney. The renal artery splits 

 up at the hilus into several branches, which pass in between the Mal- 

 pighian pyramids, and form at the boundary of the cortex and medulla 

 vascular arches, from which spring, on the one side, interlobvlar arteries 

 running up into the cortex between the pyramids of Ferrein, and, on 

 the other, vasa recta running down into the boundary layer of the 

 medulla (Fig. 188). The interlobular arteries give off at intervals 

 afferent vessels. Each of these soon breaks up into a glomerulus or tuft 

 of vascular loops, which gather themselves up again into a single 

 efferent vessel of somewhat smaller calibre than the afferent. The 

 glomerulus is fitted into a cup or capsule (of Bowman), which is closely 



