210 THE BODY AT WOEK 



When the presence in it of urea demands a more copious flow, 

 the hydrostatic conditions are adjusted to tins need. In the 

 case just cited of the isolated kidney, it might be urged that the 

 flow caused by urea is a mechanical effect. The cells of the 

 contorted portions of the urinary tubules remove urea from the 

 blood. They secrete it into the tubules. The solution of urea, 

 being headed up towards the glomeruli, owing to the resistance 

 offered to its passage down the tubules by the narrow, 

 descending limbs of Henle's loops, surrounds the capillary 

 tuft. Urea rapidly attracts water from the blood. A 

 copious flow is the result. But it is just this contrast between 

 the capacity of removing urea possessed by living cells, and the 

 passage of urea in solution from one side to the other of a mem- 

 brane, which justifies the retention of the expression " vital." 

 Mechanical conditions are those which we can imitate in a 

 model ; vital conditions, those which at present we are unable 

 to reproduce. 



Nitrogenous Waste. Meat, fish, eggs, milk, vegetable albu- 

 mins, are the sources of nitrogen. The kidney is the organ which 

 eliminates it from the body. Since all nitrogenous food which 

 is digested is eventually reduced to simple, soluble compounds 

 which appear in the urine (the quantity thrown off in perspira- 

 tion is so small as to be negligible), the proportion which the 

 nitrogen of the urine bears to the nitrogen in the food is a 

 measure of the efficiency of digestion. A certain quantity 

 of the nitrogen eliminated is in the form of uric acid, creatinin, 

 and other compounds of a like order ; but these less oxidized 

 substances, though always present in some degree, are not, in 

 Man and other mammals, the normal end-products of nitro- 

 genous metabolism. Urea is the final and simplest product. 

 It is therefore sufficient to estimate the quantity of urea 

 excreted, and to compare the nitrogen which it contains with 

 the nitrogen ingested in the form of " animal food." About 

 nine-tenths of the nitrogen ingested should be accounted for by 

 urea. When alimentation is excessive or digestion imperfect, 

 the proportion is less than this ; some nitrogenous food is not 

 absorbed ; some that is absorbed is imperfectly oxidized. 



Urea is characteristically an animal product. Inorganic 

 chemistry deals with stable, organic chemistry with unstable, 

 compounds. Not that there is any boundary between inorganic 



