The size of renal calculi may vary from almost microscopic grains, which then usually take the collective name of sand or gravel, and are most commonly composed of uric acid, up to masses of some ounces in weight, completely filling a dilated pelvis. It is doubtful in what way renal calculi originate, their constituents being always present in the urine, but rarely crystallizing out. The uric-acid infarction of new-born children can hardly be considered as accounting for any large number of cases, although it might be the basis of calculi in young children. The uric and phosphatic deposits sometimes found in the tubes of the more mature kidney may possibly, when dislodged, be a point upon which additional quantities of the same substances are deposited, but anything which delays in the pelvis or in some of its calices a concentrated urine, especially if much mucus be present, may be regarded as favoring the agglomeration of deposits. A previous pyelitis is perhaps the usual cause of phosphatic deposits. Small uric-acid calculi may sometimes be found in considerable numbers in the sulcus surrounding some of the papillæ, and of a size which could hardly afford any marked symptoms in passing down the ureter. These, if any inflammation were to arise, would form a mass with pus or mucus which might serve as a nucleus for a phosphatic calculus. These suppositions are, however, rather theoretical and fragmentary, and do not cover all the cases. Constitutional predisposition has been much discussed, though not a great deal is known about it. A gouty tendency, however, undoubtedly favors the production of uric-acid calculi.