408 A TEXTBOOK OF PHYSIOLOGY 



no cupric salt left, for a cupric salt, when brought into contact with 

 the ferrous thiocyanate, oxidizes it to ferric thiocyanate, which is 

 red in colour. A modified Fehling's solution (Pavy's solution) has 

 strong ammonia added to it. As this keeps the red precipitate of 

 cuprous oxide in solution, the disappearance of the blue colour is 

 evident. The end-point is reached when the solution becomes colour- 

 less. Pavy's solution is ten times less strong than Fehling, 1 c.c. 

 being reduced by 0-005 gramme of dextrose. The urine should 

 therefore be diluted more before carrying out the test. 



In another modification of the test (Gerrard's), the red deposit 

 of the Fehling's solution is held in solution by the double cyanide of 

 potassium and copper. Gerrard's solution is mixed and boiled with 

 Fehling's solution. In Bertrand's modification, the cuprous oxide 

 precipitate produced by the presence of the sugar is dissolved in a 

 solution of ferric sulphate in sulphuric acid. During the process of 

 solution, an amount of ferrous salt becomes produced, which is equiva- 

 lent to the amount of cuprous oxide. The ferrous salt is then 

 measured by titrating with a standardized solution of potassium 

 permanganate. It is the most reliable method for estimating sugar 

 in simple solutions. 



The polarimeter may also be employed to estimate the amount 

 of sugar in urine, but it is necessary that the urine be absolutely clear, 

 and not high-coloured. Protein, if present, and any other levo- 

 rotatory substances, must be removed before the measurement is made. 

 In diabetic urine, various volatile acids, such as 3-oxybutyric 

 acid and aceto-acetic acid, are Avont to occur in conjunction with 

 acetone. The acetone bodies are also excreted in starvation or on 

 a diet of fat with a limited amount of protein, in certain fevers, severe 

 anaemias, and phosphorus -poisoning conditions in which the tissues 

 are unable to use glucose. These bodies are closely related, as their 

 formulae show, the bodies with the smaller molecule being derived 

 from those with the larger: 



p-oxybutyric acid, CH 3 .CHOH.CJUCOOH. 

 Aceto-acetic acid, CH 3 .CO.CH 2 .COOH. 

 Acetone, CH 3 .CO.CH 3 . 



They are most probably derived from an incomplete breaking 

 down of amino-acids, particularly in the liver, and are therefore 

 indirectly derived from protein (see p. 423). In starvation and in the 

 last stages of diabetes, in diabetic coma, they are probably also derived 

 from the higher fatty acids of the body fat. 



/3-Oxybutyric Acid, a volatile acid, is levo-rotatory, and does not 

 ferment with yeast. No satisfactory simple test has been devised 

 for its detection in urine. It generally occurs combined with am- 

 monia, and an idea of the amount of the volatile acids of the urine 

 can be obtained by the method already given (p. 454). 



Aceto-acetic Acid may be tested for by adding ferric chloride to the 

 urine. If present, a red colour is obtained ; if small in amount, it may 

 be first extracted with ether, to which a little sulphuric acid is added. 



