HEIDENHAIN' S THEORY. 653 
4. The activity of the two kinds of secretory cells is determined — 
(a) By the amount of water or urinary constituents contained in the 
blood ; 
(b) By the velocity of the blood-flow through the capillaries of the 
kidney, inasmuch as on this factor depends the supply of oxygen, and 
of substances to be excreted, to the cells. 
5. The great variability in the constitution of the urine may be 
explained by differences in the secretory activities of these two types of 
cell. 
The most important part of these conclusions of Heidenhain is a 
revival of Bowman's theory, that the specific urinary constituents, urea 
and uric acid, are secreted by the tubules, and that the office of the 
tubules is secretory rather than absorbent. What evidence have w T e of 
the secretory activity of the cells in the tubules ? 
The great solubility and diffusibility of urea render it impossible to 
trace this substance on its way through the kidney by micro-chemical 
means. A better prospect of success would seem to be afforded by 
the more insoluble uric acid and urates ; and both Bowman 1 and v. 
Wittich 2 have described the presence of uric acid crystals in the cells 
of the convoluted tubules of birds. Semicrystalline deposits of guanin 
have been demonstrated with certainty in the cells of the excretory 
organ of molluscs, but later researches by Adolph Schmidt 3 have shown 
that the observations of Bowman and v. Wittich must have been due 
to faulty methods of preparation. Deposited urates were frequently to 
be seen in the urinary tubules of birds, but never in the cells them- 
selves. In order to throw light upon this point, Heidenhain had 
recourse to a method, devised by Chrzonzsczewsky, 4 i.e. the injection of 
sodium sulphindigotate (indigo-carmine) into the blood, and the tracing 
of this coloured substance through the cells of the kidney. 
It is found that this substance is excreted in any quantity by two 
glands only of the body, namely, the liver and the kidney. If 5 c.c. 1 if 
a saturated watery solution of the sulphindigotate be injected into the 
veins of a rabbit, within a few minutes the urine becomes a deep blue, 
and on killing the animal the kidneys are found to be stained blue, the 
colour being best marked towards the apex of the pyramid. In order to 
find out in what portion of the secreting substance of the kidney the 
colouring matter is turned out, the flow of urine must be checked, since 
otherwise the excreted pigment is at once washed down into the lower 
parts of the tubules and ureter. To this end Heidenhain 5 divided the 
spinal cord in the neck. The flow of urine being thus stopped, 5 c.c. of 
the saturated solution of indigo-carmine is injected into the blood vessels ; 
ten minutes later the animal is killed, and the blood vessels of the 
kidney washed out with absolute alcohol. By this means the pigment 
is precipitated in situ. On cutting into the kidney, it is at once seen to 
differ widely in appearance from that of an animal in which the cord 
was intact. Instead of being diffusely stained, the kidney now is coloured 
a deep blue in the cortex, the medulla presenting the normal appearance. 
On examining a section under the microscope, it is seen that the blue 
colour is due to the deposition of pigment granules in the lumen and 
in the striated cells lining the convoluted tubules and the ascending 
1 Loc. tit. " Arch. f. mikr. Anat., Bonn, 1875, Bd. xl. S. 81. 
3 Arch./, d. ges. Physiol., Bonn, 1890, Bd. xlviii. S. 34. 
4 Virchow's Archiv, 1866, Bd. xxxv. S. 158. 5 Loc. tit. 
