DISEASES OF THE UKINARY ORGANS. 157 



stantly formed tlie aminonio-magnesiuin phosphate, which is as promptly 

 deposited in the solid form. The common source of ammonia in such 

 cases is froni decomposition of the urea in fermenting urine. But in 

 order to produce this a ferment is necessary, and therefore, as an addi- 

 tional prerequisite, the presence of bacteria or fungi in the urine is es- 

 sential. These ferments may make their way from without along the 

 urinary passage (urethra), and their propagation in the bladder is 

 greatly favored by the prolonged retention of urine, as in case of spasm 

 of the neck of the bladder or obstruction by an already existing stone. 

 Another mode of entrance of the ferment is an uncleanly catheter used 

 to draw off the urine. 



Another insoluble salt which enters largely into the composition of 

 many urinary calculi of the ox is carbonate of lime. This is derived 

 mainly from the lime in the food and water and from the carbon diox- 

 ide formed by the oxidation of the organic acids in the fodder. These 

 organic acids, being composed of carbon, hydrogen, and oxygen (with- 

 out nitrogen), are resolved, by the addition of oxygen, into carbon di- 

 oxide (CO 2 ) and water (H 2 O). The carbon dioxide unites with the lime 

 in the blood to form carbonate of lime, and in this state passes into the 

 urine. Now, carbonate of lime is soluble in water containing free or 

 tmcombined carbon dioxide, but is precipitated whenever the latter is 

 withdrawn. It is only necessary, therefore, to have in the urine suffi- 

 cient lime or other available base to unite with all the free carbon di- 

 oxide in order to bring about the precipitation of the dissolved carbon- 

 ate of lime in the solid crystallized form. Hence it is that of all sedi- 

 ments in the urine of herbivora this is the most frequent and usually 

 the most abundant. 



A less common constituent of urinary calculi is the insoluble oxalate 

 of liuie. 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 dioxide. The final product of the complete oxida- 

 tion of these acids is carbon dioxide, 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 conies 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 (SiOi). 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 burnt plant. It is found in the 

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

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

 in the stem. This gritty, insoluble silica is especially noticeable among 



