388 



A MANUAL OF PHYSIOLOGY 



(Fig. 116 ; Plate IV., i). Calcium oxalate may also be thrown down 

 as 'envelope,' a, b, or, less frequently, 'sand-glass' crystals, c (Fig. 117). 

 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 

 urea, Bacterium urea, and others) which reach it from the air, and 



FIG. 116. URIC ACID. 



FIG. 117. CALCIUM OXALATE. 



produce a soluble ferment, in whose presence the urea is split up 

 under absorption of water. Thus : 



CON 2 H 4 



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. 118), along with amorphous earthy phosphates, and 



often acid ammonium urate in 

 the form of dark balls occa- 

 sionally covered with spines 

 _. (Plate IV., 2). 



^5j\ ^^%^J Xj\ k * s on ty m pathological con " 



^4 ^/ ditions that this alkaline fermen- 



tation takes place within the 



FIG. IIS.-TRIPLE PHOSPHATE. bladder " The reaction of the 



urine can readily be made alka- 

 line by the administration of alkalies, alkaline carbonates, or the 

 salts of vegetable acids like malic, citric, and tartaric acid, which 

 are broken up in the body and form alkaline carbonates with the 

 alkalies of the blood and lymph. It is not so easy to increase the 

 acidity of the urine, although mineral acids do so up to a certain 

 limit. If the administration of acid be pushed farther, ammonia is 

 split off from the proteids, and is excreted in the urine as the 

 ammonium salt of the acid. 



Urea, CO(NH 2 ) 2 , is the form in which by far the greater part of 

 the nitrogen is discharged from the body. Its amount is as im- 

 portant a measure of proteid metabolism as the quantity of carbon 

 dioxide given out by the lungs is of the oxidation of carbonaceous 

 material. It is soluble in water and in alcohol, and crystallizes from 



