548 THE URINARY APPARATUS [CH. XXXVI. 



place in the tubules. Cushny finds this may occur to some extent, but the main 

 function of the tubules is undoubtedly secretion, not absorption. 



The work done by the kidney cells in order to separate from the blood-plasma 

 a fluid with the much higher osmotic pressure of urine can be estimated. 



We may take some examples from Dreser's work. He took the case in which 

 200 c.c. of urine were excreted during a night ; the blood plasma in this case had an 

 osmotic pressure = 0*92 per cent solution ; while that of the urine was = 4 '0 per cent, 

 solution of sodium chloride. To measure the work of the kidney we may take the 

 mean of '92 and 4 as the average concentration of the salt during the process of 

 excretion, and thus it is found that the kidney did about 18 kilogramme-metres of 

 work. In another case of more concentrated urine obtained from a cat previously 

 deprived of water for three days, the numbers were respectively 1-1 and 8*0. The 

 difference was equal to a pressure of 498 metres of water ; so that at the end the 

 kidney had separated urine from the blood against a pressure of 49,800 grammes 

 per square centimetre, a force about six times greater than the maximum force of 

 muscle. 



Extirpation of the Kidneys. 



Extirpation of one kidney for stone, etc., is a common operation. 

 It is not followed by any untoward result. The remaining kidney 

 enlarges and does the work previously shared between the two. 



Extirpation of loth kidneys is fatal ; the urea, etc., accumulate in 

 the blood, and the animal dies in a few days; ursemic convulsions 

 (see p. 556) do not occur in such experiments. 



Ligature of "both renal arteries amounts to the same thing 

 as extirpation of the kidneys, and leads to the same result. If the 

 ligature is released the kidney once more sets to work, but the 

 urine secreted at first is albuminous, owing to the epithelium having 

 been impaired by being deprived for a time of its blood supply. 



Removal of one kidney, followed at a later period by removal of a 

 half or two-thirds of the other, leads in dogs, in which the operation 

 has been performed by Bradford, to a surprising result. After the 

 second operation the urine is increased in amount, and the quantity 

 of urea is much greater than normal. This comes from a disintegra- 

 tion of the nitrogenous tissues ; the animal wastes rapidly and dies 

 in a few weeks. It is thus evident that the kidneys play an important 

 role in nitrogenous metabolism apart from merely excreting waste 

 substances. The exact explanation has still to be found, but it is 

 possible that the kidney, like the pancreas and liver, and many duct- 

 less glands, forms an internal secretion (see p. 328). 



The Passage of Urine into the Bladder. 



As each portion of urine is secreted it propels that which is 

 already in the uriniferous tubes onwards into the pelvis of the 

 kidney. Thence through the ureter the urine passes into the bladder, 

 into which its rate and mode of entrance has been watched in cases 

 of edopia vesicce, i.e. of such fissures in the anterior and lower part of 



