[Footnote 17: Marquezy, _Thèse de Paris_, 1856.] The changes which take place are those of atrophy. Interstitial suppurative nephritis seems to follow this form of pyelitis much less frequently than that which is due to extension upward of disease in the lower urinary passages. Corresponding to the pressure of solid or fluid, the papillæ are eroded and the straight tubes shortened. In the cortical substance, which soon becomes diminished in thickness, the interstitial tissue is hypertrophied, dense, and hard, while the tubes become smaller or in time disappear. The Malpighian bodies are changed to dense masses of connective tissue, but are still plainly recognizable, irregularly crowded together instead of being arranged as usual in more or less symmetrical double rows. The cortex of the kidney may thus become but little more than a mere skin stretched over a large stone, with perhaps here and there a piece of renal structure recognizable and in a comparatively normal condition. The extremer grades of hydro-nephrosis do not seem to be met with in this form of atrophy, but the pelvis is considerably dilated, while its internal capacity is also added to by the atrophy of the renal substance. The interior of the cyst thus formed usually retains distinct traces of its original division into infundibula, and may be, as already stated, almost filled by the calculus. Kidneys undergoing this process of degeneration {49} often furnish up to a short time before death a normal, or even more than normal, amount of urine, and one is often astonished to find how little disturbance of elimination has been caused in cases where the true kidney-structure seems to the naked eye to have been almost entirely destroyed.