438 



A MANUAL OF PHYSIOLOGY 



The titratable acidity of urine (see p. 24) is chiefly due to the acid 

 (monobasic) phosphates, such as acid sodium phosphate (NaH 2 PO 4 ), 

 but in an important degree also to organic acids. According to 

 Folin, indeed, the organic acidity may be more than half the total 

 acidity. Normally the acidity diminishes distinctly, or even gives 

 place to alkalinity, during digestion, when the acid of the gastric 

 juice is being secreted. This is sometimes fancifully denominated the 

 alkaline tide. After a fast, as before breakfast, the opposite condition, 

 the acid tide, occurs. 



The acidity varies with the quantity of vegetable food in the diet. 

 The urine of herbivora and vegetarians is alkaline, and is either turbid 

 when passed, or on standing soon becomes turbid from precipitated 

 carbonates and phosphates of earthy bases, while that of carnivora 

 and of fasting herbivora, which are living on their own tissues, is 

 strongly acid and clear. Normal human urine may deposit urates 

 soon after discharge, as they are more soluble in warm than in cold 

 water. They carry down some of the pigment, and therefore usually 

 appear as a pink or brick-red sediment. When urine is allowed to 

 stand after being voided, what is generally described as ' acid fer- 

 mentation ' occurs. The acidity gradually increases ; acid sodium 

 urate is produced from the neutral urate, and comes down in the 

 form of amorphous granules, while the liberated uric acid is deposited 



FIG. 160. URIC ACID. 



FIG. 161. CALCIUM OXALATE. 



often in ' whetstone ' crystals, coloured yellow by the pigment (Fig. 

 1 60). Calcium oxalate may also be thrown down as ' envelope,' a, b, 

 or, less frequently, ' sand-glass ' crystals, c (Fig. 161). If the urine 

 is allowed to stand for a few days, especially in a warm place, or in a 

 place where urine is decomposing, the reaction becomes ultimately 

 strongly alkaline, owing to the formation of ammonium carbonate 

 from urea by the action of micro-organisms (Micrococcus urece, Bac- 

 terium urea, and others) which reach it from the air, and produce a 

 soluble ferment, in whose presence the urea is split up with assump- 

 tion of water. Thus : 



CON 2 H 4 +2H 2 O 



Urea. 



- (NH 4 ) 2 C0 3 . 



Ammonium carbonate. 



The substances insoluble in alkaline urine are thrown down, the 

 deposit containing ammonio-magnesic or triple phosphate, formed by 

 the union of ammonia with the magnesium phosphate present in fresh 

 urine, and precipitated as clear crystals of ' knife-rest ' or ' coffin-lid ' 

 shape (Fig. 162), along with amorphous earthy phosphates, and often 



tion as a standard. Fehling's solution is employed because it is a blue 

 liquid of definite depth of tint already prepared in every physiological 

 laboratory. 



