STRUCTURE AND FUNCTION OF THE KIDNEYS. 373 



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

 chief constituent of certain calculi. 



Of the other abnormal constituents of the urine which were men- 

 tioned on p. 364, it will be unnecessary to speak at length in this work. 



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

 in a state of solution. It consists of carbonic acid (chiefly) and nitro- 

 gen and a small quantity of oxygen. 



The Method of the Excretion of Urine. 



The excretion of the urine by the kidneys is believed to consist of 

 two, more or less distinct processes viz., (1.) of Filtration, by which 



FIQ. 259. Crystals of Calcium Oxalate. 



the water and the ready-formed salts are eliminated; and, (2.) of True 

 Secretion, by which certain substances forming the chief and more im- 

 portant part of the urinary solids are removed from the blood. This 

 division of function corresponds more or less to the division in the func- 

 tions 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 glo- 

 meruli. 



The greater the blood-pressure in the arterial system generally, and 

 consequently in the renal arteries, the greater, cceteris paribus, will be 

 the blood-pressure in the glomeruli, and the greater the quantity of 

 urine separated ; but even without increase of the general blood-pres- 

 sure, if the renal arteries be locally dilated, the pressure in the glo- 

 meruli will be increased and with it the secretion of urine. All the 

 numerous causes therefore, which increase the general blood-pressure 

 (p. 151) will, as a rule, secondarily increase the secretion of urine. Of 

 these 



