39 



A MANUAL OF PHYSIOLOGY 



contains benzoic acid, or substances which can be readily changed 

 into it (such as cinnamic and quinic acids), causes an increase of the 

 hippuric acid in urine. In fact, one of the best ways of obtaining the 

 latter is from the urine of a person to whom benzoic acid is given 

 by the mouth ; the sweat may also in this case contain a trace of 

 hippuric acid. Chemically it is a conjugated acid formed by the 

 union of benzoic acid and glycin. Thus : 



C 7 H6O 2 + C 2 H*NO 2 = CoHsNOg + H 2 O. 



Benzoic Acid. Glycin. Hippuric Acid. Water. 



Benzoic acid, therefore, meets glycin in the body, and combines with 

 it, as fatty acids meet glycerine and combine with it. But neither 

 free glycin nor free glycerine have been detected in the blood or 

 tissues (p. 428). 



Oxalic acid is always present, although in very small amount 

 Some of it comes from the oxalates of the food, but a portion of it 

 arises in the metabolism of proteids, and under pathological condi- 

 tions, of carbohydrates. Calcium oxalate crystals are often seen in 

 urinary sediments. 



Kreatinin (C 4 H 7 N 3 O) has only been found as a constant con- 

 stituent in the urine of man and a few other mammals. It is possibly 

 the form in which some of the kreatin of muscle leaves the body. 

 But a large portion of the urinary kreatinin is undoubtedly derived 

 from the kreatin of the meat taken as food (p. 445). Its formula 

 differs from that of kreatin only in possessing the elements of one 

 molecule of water less ; and kreatinin can be obtained by boiling 

 kreatin with dilute sulphuric acid, then neutralizing with barium car- 

 bonate, filtering, evaporating the filtrate to dryness on the water-bath, 

 and extracting the residue with alcohol. From its alcoholic solution 

 it crystallizes in colourless prisms. It forms crystalline compounds 

 with zinc chloride and other salts (p. 428). 



Pigments of Urine. The pigments of urine have not hithertc 

 been exhaustively studied ; but we already know that normal urine 

 contains several, and pathological urines probably additional, pig- 

 mentary substances. The best-known pigments in normal urine are 

 urochromc, the yellow substance which gives the liquid its ordinary 

 colour ; uroerythrin^ the pink pigment which often colours the 

 deposits of urates that separate even from healthy urine; and 

 urobilin^ sometimes termed normal urobilin^ to distinguish it from 

 the so-called febrile urobilin, which, as has been already mentioned, 

 is identical with the faecal pigment stercobilin, and occurs not only 

 in many febrile conditions, but also in cases with no fever, such as 

 functional derangements of the liver, dyspepsia, chronic bronchitis, 

 and valvular diseases of the heart. Normal and febrile urobilin are 

 said to show certain spectroscopic differences, but are probably one 

 and the same substance, and represent simply the portion of the 

 stercobilin which is not excreted with the faeces, but absorbed from 

 the intestine into the blood. The urobilin in normal urine only 

 exists in small amount in the fully-formed condition, most of it being 

 present as a chromogen or mother substance, which by oxidation, as 

 on standing exposed to the air, is converted into urobilin. 



