372 HAND-BOOK OF PHYSIOLOGY. 



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 oxidation 

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

 together in the same urine, makes it seem probable that they have differ- 

 ent 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, although 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 evidence for 

 believing that uric acid is an antecedent of urea in the nitrogenous 

 metabolism of the body. Some experiments seem to show that uric acid 

 is formed in the kidney. 



Hippuric Acid (C 9 H 9 N0 3 ). Hippuric acid is closely allied to benzoic 

 acid; and this substance when introduced into the system, is excreted by 

 the kidneys as hippuric acid (lire). Its source is not satisfactorily deter- 

 mined: in part it is probably derived from some constituents 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 li< ng on an exclu- 

 sively animal diet. Hippuric acid arises from the union of benzoic acid 

 with glycin (C 2 H 5 N0 2 + C 7 H 6 O 2 = C 9 H 9 N0 3 + H 2 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 probably 

 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 metab- 

 olism of the nitrogenous food, as when this is diminished, it diminishes, 

 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 tubes onward 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 gf the walls of the abdomen, and of the front 



