THE FUNCTION OF THE TUBULES 719 



kidney took forty-six hours to reduce the osmotic pressure of the 

 blood to normal, and that in so doing the kidney performed 1 

 million grm. cms. of work ; but if the dog was given water to drink 

 the same result was produced in half the time with half the 

 expenditure of work. There is really nothing in these experiments 

 which shows by what mechanism the glomerulus brings about 

 the early diuresis, nor whether the subsequent rise in the 

 molecular concentration of the urine is caused by the reabsorption 

 of water to meet the needs of the body or by the excretion of 

 solids together with a minimal quantity of water. 



From all the above considerations we can conclude only that 

 there is no evidence which decides whether or not the capillary 

 pressure is a force adequate to produce urine by filtration, but on 

 the whole the evidence would seem to be against such an idea. 



The Function of the Tubules. We have incidentally dealt 

 with the problem whether the tubules are an excretory or absor- 

 bent mechanism, but we must now turn to the experiments which 

 have been specially directed to the solution of this problem. 



Injection of Dyes. Efforts have been made to decide this 

 question directly by tracing the course through the kidney of 

 some substance which could be recognised microscopically. Since 

 none of the normal constituents of the urine can with any 

 certainty be recognised within the renal cells, Heidenhain had 

 recourse to indigo- carmine, which after intravenous injection 

 rapidly appears in the urine. His method of experiment is as 

 follows. The spinal cord is cut in the cervical region in order to 

 stop the flow of urine, and so prevent the pigment from being 

 washed at once down the tubules ; then 5 c.c. of a saturated 

 solution of indigo -carmine are injected into a vein ; ten minutes 

 later the animal is killed and absolute alcohol is rapidly forced 

 through the renal artery in order to precipitate the pigment in 

 situ. On splitting the kidney open it is seen that the cortex 

 is deep blue but the medulla is unstained. Microscopical sections 

 show pigment granules in the lumen of the tubules, in the cells of 

 the convoluted tubules and the ascending loops of Henle, but 

 none in the glomeruli or the cells of other parts of the tubule. 

 Heidenhain concluded from this result that the cells of the 

 convoluted tubules and the ascending loops of Henle excreted 

 indigo-carmine from the blood, and had, therefore, normally an 

 excretory function. This experiment, however, is inconclusive 



