1124 PHYSIOLOGY 



(2) On heating with dilute alkali, aceto-acetic acid is decomposed, with the pro- 

 duction of acetone. This may be detected by its odour or by distilling off a small 

 proportion of the fluid and testing the distillate in the following ways : 



(a) On the addition of sodium hydrate and iodine and warming, iodoform is formed. 



(6) Legal's test. A few drops of freshly prepared sodium nitroprusside solution is 

 added and the mixture rendered alkaline with sodium hydrate. A deep red colour is 

 formed. On acidifying with acetic acid this colour is changed to a reddish purple. 



CYSTINE. This substance, which is a normal product of the hydrolysis of proteins, 

 is found as a constant constituent to the amount of half a gramme a day in the urine of 

 certain individuals. The condition of cystinuria represents, like alcaptonuria, an inborn 

 error of metabolism. It is found in the child and persists throughout life. In such 

 cases the cystine may give rise to urinary deposits or even to a urinary calculus. 



HOMOGENTISIC ACID. This is an aromatic acid having the composition of 

 dioxyphenyl acetic acid. Its formula is as follows '. 



OH 



A 



CH 2 .COOH 



OH 



It occurs as a consitutent of the urine of certain individuals, who are said to be 

 affected with alcaptonuria. The urine of these cases is remarkable for its resistance to 

 putrefactive changes. It slowly darkens on exposure to the air, and on the addition of 

 alkali and shaking with air it becomes rapidly brown or black. It reduces Fehling's 

 solution, so that the presence of sugar may be suspected. Such urine contains homogen- 

 tisic acid in a quantity of 3 to 6 grm. per day. The amount of the acid excreted varies 

 with the protein food taken. It seems that in these cases the power of the organism to 

 break up tyrosine and phenylalanine is entirely absent. If either of these substances be 

 administered by the mouth it is converted almost quantitatively into homogentisic acid, 

 which appears in the urine. Individuals with alcaptonuria continue to secrete homo- 

 gentisic acid during starvation, so that the tyrosine and phenylalanine set free in the 

 course of tissue disintegration undergo the same fate as when they are derived from the 

 food. Alcaptonurics apparently suffer no ill effects as a result of their abnormal 

 metabolism. The tyrosine and phenylalanine can be absorbed and play their part 



in building up the proteins of the tissues, 

 but the process or ferment is wanting which 

 is responsible for the further break-up 

 of the first product of their oxidation, 

 namely, homogentisic acid. 



URINARY DEPOSITS 

 In addition to formed elements, 

 such as blood-corpuscles, bacteria, or 

 pus- cells, which may occur in abnor- 

 ,-, _ 00 ' . mal urine, the following deposits may 



FIG. 523. Various forms of uric acid 



crystals. (FREY.) be lOUnd : 



(a) IN ACID URINE 



(1) Amorphous urates occur generally as a brick-red amorphous deposit 

 thrown down as the urine cools. It is redissolved on warming the urine, and 

 consists generally of the quadri- urates. The acid urate of sodium and of 

 ammonium may occasionally occur in star-shaped clusters of needles or as 

 spherules with small crystals adhering to them. 



