V THE SECRETION OF URINE 187 



in warm weather and especially when the skin is active 

 and perspiring, less will go to the tidney, and the 

 secretion of urine will be small. On the other hand, if 

 the blood be largely cut off from the skin, as in cold 

 weather, more blood will be thrown upon the kidney and 

 more urine will be secreted. Thus the skin and the 

 kidneys play into each other's hands in their efforts to get 

 rid of the superfluous water of the body. 



But the whole of the urine is not thus secreted, through 

 a sort of filtering process, by the Malpighian capsules. 

 The circulation in the kidney is peculiar, inasmuch as the 

 blood coming from the glomeruli is not sent at once into 

 a vein, but is carried into a second capillary network, 

 wrapped round the tubules. The tubules are lined, as 

 has been stated, by epithelium cells, and these cells, in 

 certain parts of the tubule, especially where these are 

 coiled, are what is called secreting cells. That is to say 

 they have the power, by some means which we do not at 

 present fully understand, to take up from the blood, 

 which is flowing in the capillaries wound round the 

 tubules, or rather from the plasma which exudes from 

 those capillaries, and bathes the bases of the cells, certain 

 substances, and to pour these substances into the cavity of 

 the tubule. 



And we have evidence that many of the most important 

 constituents of the urine, such as urea, uric acid and 

 others, are thus secreted by the epithelium cells of the 

 tubules, and not simply filtered ofi" by the Malpighian 

 capsules. 



We may give two striking facts in support of this view. 

 In some animals the glomeruli of the kidney receive their 

 blood-supply by an artery, which is quite distinct from 

 the vessel which takes blood to the tubules. When the 

 artery supplying the glomeruli is tightly tied, no blood 

 can go to the glomeruli, but urea is still passed out from 

 the kidney and must come from the tubules. Again, a 

 certain colouring matter when injected into the blood is 

 excreted in the urine ; this colouring matter can easily be 



