The most frequent constituents of urinary calculi are phosphates, uric acid and oxalate of lime. Phosphates are precipitated from alkaline urine, uric acid and oxalate of lime from acid urine. Phosphates are deposited abundantly when urine, after being passed, undergoes its usual alkaline decomposition; uric acid and oxalates are thrown down in crystals when the urine, at the time of evacuation or afterwards, is unduly acid, the deposition of oxalates occurring usually some time after the urine has been passed, so that this precipitate often, as it were, powders the surface of other deposits.

Inside the bladder or the pelvis of the kidney, phosphates may be deposited because of undue alkalinity, especially in the case of alkaline decomposition of the urine. These often form an external coating on other stones when, from decomposition, the urine has become alkaline. Phosphates are also frequently deposited on foreign bodies which have found their way into the bladder.

The precipitation of Uric acid and Oxalate of lime seems sometimes due to their excess in the urine. This applies especially to the oxalates, there being individuals in whom an excess of oxalate of lime is almost always present in. the urine (oxalic acid diathesis). But in both cases the substances are deposited from an acid urine, and in the case of uric acid it must be strongly acid. The cause of this abnormal acidity is not always clear. In some cases it may be due to excessive development of acid in the stomach.

The formation of calculi of uric acid and oxalate of lime seems always to begin in the kidney, and even in some cases in the uriniferous tubules. In the tubules and in the pelvis it is common to find small concretions of uric acid, forming the so-called Sand and gravel, composed of aggregates of uric acid crystals, usually rounded in shape and of a brownish red colour. There may even be large concretions of uric acid in the pelvis of the kidney. In connection with the formation of the uric acid and oxalate calculi it is interesting to observe that the two bodies are often combined in one calculus, the mulberry stone especially having frequently a nucleus of uric acid, and perhaps layers of it alternating with the oxalate of lime.

Forms Of Calculi

We give here a brief description of the different forms of calculi, with an indication of the chemical methods for determining their constitution. As a rule it is convenient to have the calculus sawn through the middle with a lapidary's saw, so that the arrangement of its layers and the characters of its nucleus may be seen.