482 HANDBOOK OF PHYSIOLOGY. 



which is frequently deposited in combination with calcium (fig. 30(5) as 

 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. 472, 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 Excretion of Urine. 



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 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 

 functions of other glands of which we have already treated. It will be 

 as' well to consider them separately. 



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 de- 

 pends almost entirely upon the blood-pressure in the glomeruli. 



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

 consequently in the renal arteries, the greater, cceteris parilus, 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 glomeruli 

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

 therefore, which increase the general blood-pressure will secondarily 

 increase the secretion of urine. Of these 



(1) The heart's action is among the most important. When the 

 cardiac contractions are increased in force or frequency, increased 

 diuresis is the result. 



(2) Since the connection between the general blood-pressure and the 

 nervous system is so close it will be evident that the amount of urine 

 secreted depends greatly upon the influence of the latter. This may be 

 demonstrated experimentally. Thus, division of the spinal cord, by 

 producing general vascular dilatation, causes a great diminution of blood- 

 pressure, and so diminishes the amount of water passed; since the local 

 dilatation in the renal arteries is not sufficient to counteract the general 

 diminution of pressure. Stimulation of the cut cord produces, strangely 

 enough, the same results i.e., a diminution in the amount of the urine 



