Fibromata in the form of small nodules are occasionally found. Lipomata and leiomyomata are sometimes encountered.

Fig. 170. - Congenital Cystic Kidney.

Only very small portions of the kidney tissue remain, crowded between the cysts (Delafield and Prudden).

Sarcoma is quite common and is either congenital or else appears, as a rule, very early in life. The tumor may attain considerable size, is usually grayish in color, although it may be red if very vascular, and may be quite soft. Generally the growth is composed of round or spindle cells, but not infrequently are found fibers of striped muscle, the so-called rhab-dosarcoma. Such growths are really mixed embryonic tumors resulting from cell inclusion, possibly of the Wolffian body. Sarcoma may occasionally be found in later life. Such a tumor differs from the congenital form in being slow in growth and not giving metastases until late. The congenital form is rapidly malignant and destructive.

Of the tumors of epithelial type, although of mesodermic origin, the hypernephroma is the most common. It is derived from portions of adrenal tissue that have been included within the kidney during its development. The structure of the tumor resembles that of the adrenal gland, except that the cortical elements are the more prominent. Adrenal rests very seldom contain any medullary tissue. The neoplasm may be small and remain circumscribed, or it may take on a rapid growth and become destructive, giving metastasis through the bloodvessels. The cells contain fat and glycogen.

Adenoma is rare. It occurs as small, circumscribed nodules composed of glandular alveoli, which may show papillary outgrowths and also at times be cystic. Occasionally it becomes malignant.

Carcinoma as a primary growth is unusual and secondary metastases are not common. The secondary form makes its appearance as small scattered nodules. The carcinoma may become quite large, and destroy the renal tissue, and is frequently associated with hemorrhages into the tissue.

Cysts of the kidney are quite common and are of various forms. They may be single or multiple, large or small. In the majority of cases they appear as numerous small collections of fluid scattered over the surface of the kidney. They are simple retention cysts due to the obstruction of the tubule below the glomerulus; are especially common in chronic interstitial nephritis. Occasionally a kidney may contain a single large cyst and otherwise be apparently normal. Is probably due to the obstruction of a tubule.

The kidneys may be congenitally cystic, and in such a case present a characteristic appearance. Both kidneys are generally affected and are filled with numerous cysts, some of which may be as large as a walnut. These may be filled with a urinous or even a colloid material, and are separated from each other by a very thin stroma of connective tissue. They probably result from a failure in the accurate joining of the urinary tubules (nephrogenic tissue) with the outgrowths of the Wolffian duct. The organs may be very much enlarged. Generally such a condition is incompatible with life, but similar lesions may be found in adults. As long as the remaining renal substance is able to maintain the excretion the individual will live, but when the organs become incompetent, death may ensue from uremia.