THE KIDNEYS AND URINE. 



365 



rides exist largely in food and in most of the animal fluids, their occur- 

 rence in the urine is easily understood. 



Cystin (C 3 H,NSO a ) (Fig. 252) is an occasional constituent of urine. 

 It resembles tauriu in containing a large quantity of sulphur more than 

 25 per cent. It does not exist in healthy urine. 



Another common morbid constituent of the urine is oxalic acid, whicli 

 is frequently deposited in combination with calcium (Fig. 253) as a 



FIG. 252. Crystals of cystin. 



FIG. 253. Crystals of calcium oxalate. 



urinary sediment. Like cystin, but much more commonly, it is the chief 

 constituent of certain calculi. 



Of the other abnormal constituents of the urine mentioned it will be 

 unnecessary to speak at length in this work. 



Gases. A small quantity of gas is naturally present in the urine in 

 a state of solution. It consists of carbonic acid (chiefly) and nitrogen 

 and a small quantity of oxygen. 



THE METHOD OF THE EXCKETION OF UKINE. 



The excretion of the urine by the kidney is believed to consist of two 

 more or less distinct processes viz., (1.) of filtration, by which the water 

 and the ready-formed salts are eliminated; and (2.) of true secretion, by 

 which certain substances forming the chief and more important part of 

 the urinary solids are removed from the blood. This division of function 

 corresponds more or less to the division in the functions of other glands 

 of which we have already treated. It will be as well to consider them 

 separately. 



(1.) Of Filtration. This part of the renal function is performed 

 within the Malpighian corpuscles by the renal glomeruli. By it not only 

 the water is strained off, but also certain other constituents of the 

 urine, e.g., sodium chloride, are separated. The amount of the fluid 

 filtered off depends almost entirely upon the blood -pressure in the 

 glomeruli. 



