Unlike hypertrophy, this is constantly the result of a pathological process. So long as a normal functional activity of the secreting elements is carried on, such parts must maintain their size and healthy characters. But with the compression of such secreting elements (glomeruli and convoluted tubes) by a hyperplasia of connective tissue, by pressure from without or from the damming back of the urine in the pelvis and tubes, the secretory elements are absorbed and removed, and the final result is a general atrophy. If such atrophy appears in both kidneys at once it can only be very partial in extent, as extreme atrophy of both, with loss of their secretory function, would entail poisoning and death from the retained urinary products. The comparative frequency of the disease may be inferred from the reports of the numbers of specimens found by Barrier and Moussu in old horses in the dissecting rooms. The latter observed a dozen cases in a single winter, other examples are recorded by Cadeac (horse), Soula (swine) and Trasbot (in various animals). _Causes._ The most common source of the condition is the occurrence of chronic productive inflammation. The new product in such cases, if not pus, or a growth that rapidly passes into fatty or granular degeneration, or into gangrene, tends to form tissue of a low organization, especially fibrous. The resulting increase of the fibrous trabeculæ, in undergoing subsequent contraction necessarily compresses the secretory tissue and the final result is a visible and, it may be, extreme wasting. Hence any slowly advancing productive inflammation is liable to result in absorption and removal of the kidney parenchyma, and distinct atrophy of the gland. Again the obstruction of the ureter by a calculus in the pelvis which falls into the infundibuliform entrance, or a stone arrested at any part of the duct (or even of the urethra) or by worms, hydatids, cysts or tumors, throws back on the kidney the secreted urine, which distending the pelvis and uriniferous tubes leads to direct compression and absorption of the secretory parenchyma. Direct compression of the kidney by an adjacent tumor will act in a similar manner. Retention cysts by their gradual increase and augmenting pressure cause absorption of the gland tissue. The blocking of individual uriniferous tubules by minute calculi, which is so often seen in cattle, kept on dry feeding in winter, is a cause of partial nephritis, and absorption, as noted by Röll. A somewhat rare cause of atrophy is the diminution of the blood supply by arteritis and embolism of the renal artery, or by pressure of tumors on that vessel. Arteritis and blocking suggests at once the possible agency of the strongylus (sclerostoma) armatus in the horse. Trasbot records a striking instance of compression of the renal artery and kidney by an enormous sublumbar melanoma. This occurred in an aged horse and led to atrophy.