On the origin of vesicular calculus there is very little of a definite nature to be advanced. It is a well-established truth that under certain local as well as general conditions of the body the renal secretion undergoes various modifications and changes both in its physical state and chemical constitution. Thus, normal constituents may be increased or diminished, or altogether disappear, while others foreign to the secretion are sometimes found entering into its composition.
These departures from the general standard are in some cases doubtless connected with physiological deviations in the complex processes of assimilation, and in some measure also with chemical alterations which the urine undergoes after its departure from the kidneys.
In diseased conditions of the system peculiar compounds are not un-frequently formed which are rarely or never produced in the healthy organism, and, being feebly soluble in urine, are immediately deposited in a solid form from that fluid. In this manner oxalate of lime conies to form a part, and in some very rare cases the whole, of the vesicular calculus in the horse.
To what extent the superabundant formation of lime-salts in the economy is referable to food, water, climate, and assimilative disturbance, separately or together, we have at present but little to guide us to a satisfactory conclusion. The fact remains, nevertheless, that some horses eliminate from their systems an amount of calcic carbonate that is simply astonishing. The writer's attention was recently called to a case in which a considerable amount of this salt was periodically removed from the bladder of a mare in addition to that which escaped with the urine in the act of micturition.
On this subject the late Professor Morton remarks: "The water drunk by animals has generally been considered as the source of calculi, but it is by no means proved that in those localities where lime is more abundantly met with in water, as Matlock, Scarborough, Carlsbad, and other limestone districts, that in these, calculous affections are most prevalent; whereas we do know that animals kept on any of the lime plants for a long time, or pastured where lime has recently been laid, become the subjects of these accumulations. Nevertheless, excess of lime in water will readily furnish the requisite calcareous matter."
Why the salts of the urine should cease to be held in solution by the urinary secretion may be conceived to arise either out of a supersaturated condition of that fluid or from chemical reactions resulting in the production of insoluble compounds, but it is not always so easy to comprehend the reasons which in certain cases determine the aggregation of small particles of salts and the development of a distinct calculous formation or stone. Such a state of quiescence as is afforded by a paralysed bladder would appear to favour the separation and aggregation of the crystallizable constituents of the urine, as would also its retention for long periods in the cavity of the bladder, either as the result of habit or by force of stricture of the urethra, prostatic enlargement, or other like interferences with its proper and due discharge, but it cannot be said that stone in the bladder is specially prevalent under these circumstances.
Experience gives no encouragement to the idea that the tendency to stone formation is greater in proportion to the amount of stone-forming salts secreted by the kidneys.