The conditions which lead to the deposit of oxalate of lime are not sufficiently well known to make the prophylaxis of this calculus easy by any chemical means, except by dilution of the urine and by a general tonic regimen with abundant exercise. Although it is not usual for a calculus to be arrested in the ureter after having once fairly entered, this sometimes occurs, and the result is stoppage of the flow of urine upon that side, dilatation of the ureter, followed in turn by dilatation of the pelvis, and finally atrophy of the {47} renal substance. This does not happen suddenly, however. The urinary passages do not rapidly dilate to any considerable extent, and their increase in calibre under pressure from within has been considered a growth rather than a distension. This condition will be treated under the head of Hydro-nephrosis. Calculous Pyelitis. When a calculus remains in the pelvis of the kidney without completely obstructing the flow of urine, it usually increases in size, while the resulting irritation may be the cause of fresh deposits either upon the surface of the original calculus or in the form of new concretions. In this way immense deposits of urinary salts may be formed. Thus, in a case given in detail in the second series of _Boston City Hospital Reports_ there was found upon the one side a calculus which when perfectly clean and dry weighed 204 grammes, filling the whole dilated pelvis and sending prolongations into the calices, so that its shape was compared to that of a hippopotamus. The resemblance was made more complete by the wrinkling and roughness of the exterior. In the other kidney were several hundred calculi, from the size (and shape) of a large almond down to that of white mustard-seed. The latter were composed of two apparently distinct substances--one a reddish-brown, looking like uric acid, and the other of the color and polish of white marble; both, however, were phosphates. The amount of local disturbance produced in the pelvis of the kidney by the presence of a foreign body seems to depend somewhat upon the character of its surface. Rough and uneven calculi, such as oxalate of lime, are apt to produce inflammation much more rapidly than smooth and polished ones, but it is seldom that any calculus remains without some pyelitis. At first only a loss of polish of the mucous membrane, with a little increase of mucus, may be observed, to which succeed roughening and suppuration with occasional fibrinous deposit. The pelvis, more or less dilated, may then contain a quantity of mucopurulent urine, with perhaps some blood, in which are concealed the stones which have given rise to this condition, and often phosphatic deposits not converted into calculi.