There are probably at least three factors concerned in the formation of calculi. In the first place, the composition of the food must play a part of some importance, but in addition to this two other factors influence the formation of calculi. First, the amount of the particular calculus forming ingredient in the urine and its chemical relationship with other urinary constituents, and secondly, the presence of some inflammatory affection, sometimes very slight in amount, in the renal pelvis. The formation of a calculus does not depend solely on the mere amount of uric acid or oxalic acid present in the urine. It is well known, for example, that urines that deposit uric acid are very often relatively dilute, and that, on the other hand, urines containing a large percentage of uric acid frequently only deposit urates owing to the excess of other urinary salts present. Thus the formation of the uric acid calculus cannot be correlated simply with the amount of uric acid present in the urine. Still the amount may influence the formation of the calculus to some degree, and hence the regulation by diet of the amount excreted so far as possible becomes important.

The part played by slight pyelitis in the production of calculi is probably of considerable importance, since the formation of a calculus may often depend on an irregular mode of cystalliza-tion, such as the formation, for example, of a large crystal under the influence of the presence of some colloid material such as blood, pus, or albumin in the urine. Doubtless the pyelitis may sometimes arise from the action of micro-organisms, and although pyelitis cannot be controlled purely by dietetic measures, yet it can be influenced to some extent by modifying the composition of the urine, especially as regards its acidity.

Where it is desirable to modify the reaction of the urine for prolonged periods, dietetic measures are more suitable than the mere administration of drugs.

Alteration in the reaction of the urine may also be of importance in influencing the formation of calculi, inasmuch as highly acid urines are more prone to deposit uric acid, for instance, than less acid urines. Although the excretion of uric acid in the urine cannot be prevented by any system of dieting, since uric acid is a product of the metabolism of the tissues and is excreted during starvation, yet the deposition of uric acid in the urine either in the form of gravel or calculi may be materially influenced by dietetic measures that act, not only by restricting the output of uric acid, but also by altering the reaction of the urine. Attention is often far too much concentrated on the quantity of uric acid in the urine, rather than on the conditions leading to its deposition.

In the case of deposition of uric acid, diet then may influence the process in at least two ways, by affecting the quantity of uric acid present, and secondly, by affecting the reaction of the urine and so either increasing or diminishing the rapidity of the deposition of uric acid, and, as already pointed out, the latter process may be the more important of the two. In the case of the deposition of oxalate of lime the determining conditions are somewhat different, inasmuch as the great bulk of the oxalic acid excreted in the urine in the form of oxalates is derived solely from the food or from products of the decomposition of the food in the alimentary canal. It is probable that at the most only traces of oxalic acid are formed during the metabolism of the tissues, and thus the amount of oxalates in the urine can be affected far more by diet than is the case with uric acid. Approximately one half of the uric acid excreted is of endogenous origin, i.e. produced by the metabolism of the tissues. The formation of calculi containing oxalates depends, however, on other factors than the mere quantity of oxalic acid excreted, since the essence of the process lies in the formation of the relatively insoluble oxalate of lime in greater or less abundance instead of the excretion of soluble oxalates. Certain articles of diet, especially rhubarb and tomatoes, contain considerable quantities of oxalates, and it is not unusual for oxaluria, due to the ingestion of considerable quantities of these substances, to give rise to symptoms such as pain and haematuria. Oxalates, however, may be formed in abundance in the stomach, especially in cases of dilatation of the stomach, and in other conditions where the secretion of hydrochloric acid is deficient and fermentation and decomposition of the gastric contents takes place. This diet may give rise to oxaluria either directly owing to the presence of considerable quantities of oxalates actually in the substance ingested, or indirectly owing to abnormal decomposition taking place as a result of fermentation of the gastric contents.

The regulation of the diet in cases of calculous disease is more important from the point of view of prevention than of cure. No dietetic measures can affect a stone already formed, but it is probable that careful regulation of the diet may prevent the formation of subsequent stones where one has been passed or removed.