388 ESSENTIALS OF PHYSIOLOGY. 



nor does it become self-reducing when kept for some time. Other tests 

 for dextrose are (1) the preparation of the osazone with phenylhydrazine 

 and (2) the fermentation test with yeast, which converts dextrose into 

 carbonic acid and alcohol. 



The estimation of sugar may . be effected by Benedict's method. The 

 solution used contains copper sulphate, sodium carbonate and citrate, 

 potassium thiocyanate, and potassium ferrocyanide. 25 c.c. of the solution 

 and 3 or 4 grams of anhydrous sodium carbonate are placed in a small 

 flask and boiled. The sugar solution is added from a burette until the blue 

 colour of the reagent disappears ; this is the end point. The thiocyanate 

 forms a white precipitate with the cuprous hydroxide formed by the reduction 

 of the copper sulphate, and the end point is quite sharp. 25 c.c. of the 

 solution are reduced by O05 grams of dextrose. 



Glycuronic acid (COOH(CHOH) 4 , CHO) sometimes occurs in urine, 

 either after the administration of certain drugs, such as chloral, or in 

 combination with indoxyl. It reduces Fehling's and Benedict's solutions 

 and forms an osazone, but may be distinguished from dextrose by means 

 of yeast, which does not ferment it. 



(3) Bile is present in the urine in jaundice, giving it a greenish or 

 brownish colour : its presence may be recognised by Gmelin's or Hay's 

 test (p. 320). 



(4) Blood occurs in urine as the result of haemorrhage in the 

 kidneys or urinary passages, and may be identified by observing the 

 red corpuscles under the microscope, or by spectroscopic examination. 



(5) Products of Abnormal Metabolism. These include /3-oxybutyric 

 acid, diacetic acid and acetone (p. 347), leucine and tyrosine which 

 are present in acute atrophy of the liver, cystine, and homogentisic 

 acid. 



Cystine occurs in proteins, and is set free when these break down in 

 the body, but is normally excreted in another form in the urine ; in 

 rare cases the cystine derived from protein in the body is excreted as 

 such, and may form crystalline deposits or calculi. 

 OH 



Homogentisic acid, CH . COOH, is a derivative of tyrosine. 



^ 



In certain persons the oxidation of tyrosine is incomplete, and stops with 

 the production of homogentisic acid, which appears in the urine. The 

 condition is known as alcaptonuria, and the urine darkens on standing 

 and reduces Fehling's solution. In persons suffering from alcaptonuria 

 the amount of homogentisic acid in the urine varies from 3 to 6 grams 

 daily; it is 'proportional to the quantity of phenylalanine and tyrosine 

 present in the proteins of the food, and is increased when these sub- 



