STRUCTURE AND FUNCTION OF THE KIDNEYS. 381 



work done by the muscles is expressed by the quantity of urea excreted 

 in the urine must without doubt be given up. 



Uric Acid. Uric acid probably arises much in the same way as urea, 

 either from the disintegration of albuminous tissues, or from the food. 

 The relation which uric acid and urea bear to each other is, however, 

 still obscure; but uric acid is said to be a less advanced stage of the oxi- 

 dation of the products of proteid metabolism. The fact that they often 

 exist together in the same urine makes it seem probable that they have 

 different origins; but the entire replacement of either by the other, as 

 of urea by uric acid in the urine of birds, serpents, and many insects, 

 and of uric acid by urea, in the urine of the feline tribe of Mammalia, 

 shows that either alone may take the place of the two. At any rate, al- 

 though it is true that one molecule of uric acid is capable of splitting up 

 into two molecules of urea and one of mes-oxalic acid, there is no evi- 

 dence for believing that uric acid is an antecedent of urea in the nitro- 

 genous metabolism of the body. Some experiments seem to show that 

 uric acid is formed, at any rate in part, in the kidney. 



Hippuric Acid (C 9 H 9 N0 3 ). The source of hippuric acid is not 

 satisfactorily determined; in part it is probably derived from some con- 

 stituents of vegetable diet, though man has no hippuric acid in his food, 

 nor, commonly, any benzoic acid that might be converted into it; in 

 part from the natural disintegration of tissues, independent of vegetable 

 food, for Weismann constantly found an appreciable quantity, even when 

 living on an exclusively animal diet. Hippuric acid arises from the union 

 of benzoic acid withglycin (C a H 5 N0 3 + C 7 H 6 2 =C 9 H 9 N0 3 + H 3 0), which 

 union may take place in the kidneys themselves, as well as in the 

 liver. 



Extractives. The source of the extractives of the urine is proba- 

 bly in chief part the disintegration of the nitrogenous tissues, but we are 

 unable to say whether these nitrogenous bodies are merely accidental, 

 having resisted further decomposition into urea, or whether they are the 

 representatives of the decomposition of special tissues, or of special forms 

 of metabolism of the tissues. There is, however, one exception, and 

 this is in the case of kreatinin ; there is great reason for believing that 

 the amount of this body which appears in the urine is derived from the 

 metabolism of the nitrogenous food, as when this is diminished, it di- 

 minishes, and when stopped, it no longer appears in the urine. 



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 ectopia vesicce, i. e., 

 of such fissures in the anterior or lower part of the walls of the abdomen, 



