96 DISEASES OF THE HOESE. 



the excretion of urea, but also of oxalic acid. If may therefore be 

 inferred that oxalic acid is not produced from the carbonaceous food 

 alone, but also from the disintegration of the nitrogenous tissues of 

 the body. An important element of its production is, however, the 

 imperfect performance of the breathing functions, and hence it is 

 liable to result from diseases of the chest (heaves, chronic bronchitis, 

 etc.). This is, above all, likely to prove the case if the subject is fed 

 to excess on highly carbonaceous foods (grass and green food gener- 

 ally, potatoes, etc.). 



Carbonate of magnesia, another almost constant ingredient of the 

 urinary calculi of the horse, is formed the same way as the carbon- 

 ate of lime — from the excess of carbonaceous food (organic acids) 

 becoming oxidized into carbon dioxide, which unites with the mag- 

 nesia derived from the food. 



The phosphates of lime and magnesia are not abundant in urinary 

 calculi of the horse, the phosphates being only present to excess in 

 the urine in two conditions — (a) when the ration is excessive and 

 specially rich in phosphorus (wheat bran, beans, pease, vetches, rape 

 cake, oil cake, cotton-seed cake) ; and (b) when, through the morbid 

 destructive changes in the living tissues, and especially of the bones, 

 a great amount of phosphorus is given off as a waste product. Under 

 these conditions, however, the phosphates may contribute to the for- 

 mation of calculi, and this is, above all, likely if the urine is retained 

 in the bladder until it has undergone decomposition and given off 

 ammonia. The ammonia at once unites with the phosphate of mag- 

 nesia to form a double salt — phosphate of ammonia and magnesia — 

 which, being insoluble, is at once precipitated. The precipitation of 

 this salt is, however, rare in the urine of the horse, though much 

 more frequent in that of man and sheep. 



These are the chief mineral constituents of the urine which form 

 ingredients in the horse's calculi, for though iron and manganese are 

 usually present it is only in minute quantities. 



The excess of mineral matter in a specimen of urine unquestion- 

 ably contributes to the formation of calculi, just as a solution of such 

 matters out of the body is increasingly disposed to throw them down 

 in the form of crystals as it becomes more concentrated and ap- 

 proaches nearer to the condition of saturation. Hence, in consider- 

 ing the causes of calculi we can not ignore the factor of an excessive 

 ration, rich in mineral matters and in carbonaceous matters (the 

 source of carbonates and much of the oxalates), nor can we overlook 

 the concentration of the urine that comes from dry food and priva- 

 tion of water, or from the existence of fever which causes suspension 

 of the secretion of water. In these cases, at least the usual amount of 

 solids is thrown off by the kidneys, and as the water is diminished 

 there is danger of its approaching the point of supersaturation, when 

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