DISEASES OF THE URINARY ORGANS. 



135 



with the lime in the blood to form carbonate of lime, and in this 

 state passes into the urine. Xow, carbonate of lime is soluble in 

 water containing free oruncombined carbon dioxid,but is precipitated 

 whenever the latter is withdrawn. It is only necessary, therefore, 

 to have in the urine sufficient lime or other available base to unite 

 with all the free carbon dioxid in order to bring about the precipi- 

 tation of the dissolved carbonate of lime in the solid crystallized 

 form. Hence it is that, of all sediments in the urine of herbivora, 

 this is the most frequent and usually the most abundant. 



A less comumon constituent of urinary calculi is the insoluble oxalate 

 of lime. In this case the lime is derived as before from the food or 

 water, or both, while the oxalic acid is a product of the oxidation of 

 organic acids of the food, less oxygen having been used than in the 

 formation of carbon dioxid. The final product of the complete 

 oxidation of these acids is carbon dioxid, but when less oxygen is 

 furnished owing to some disease of the lungs or a disease of the nerve 

 centers, which lessens the activity of the breathing, then oxalic acid 

 may be produced. Then if this oxalic acid comes in contact with 

 lime, it is instantly precipitated as crystals of oxalate of lime. 



Another inorganic substance at times found in urinary calculi is 

 silica (SiO,). This contributes largely to giving stiffness to the 

 stems of growing plants, and in most of our cereals and grasses makes 

 up a large proportion of the ashes of the burned plant. It is found in 

 the soluble form in combination as silicate of potash, but at times is 

 displaced by oxalic or other acid and then appears as gritty, sandy 

 particles in the stem. This gritty, insoluble silica is especially notice- 

 able among the horsetails {E quisetacece) ^ bamboos, and sedges. The 

 percentage of silica in the ash of several common fodder plants is 

 given below : 



Silica in ash of various fodder plants. 



It is only soluble silica that is taken up into the .system, and it 

 is in this form (usually as silicate of potash or soda) that it enters 

 the urine, but all that is wanted to precipitate it in crystalline fonn 

 as a gritty sand is the presence of oxalic or other acid having a 

 stronger affinity for its base (potash or soda). 



