Fatty degeneration of the kidney occurs un der two distinct forms. In the first form of the disease in question, the kidneys are usually large, smooth, soft, pale, and mottled, and frequently they are scattered over with hmmorrhagic spots. On a microscopical exa mination, there is found to be a great increase in the size and number of the ,oil globules which exist in small quantities in the epithe lial cells of the healthy gland. (See .fig. 164.) The urinary tubes are filled and distended by the gorged epithelial cells, the dilated tubes compress the capillary plexus on their exterior, and hence, in consequence of passive conges tion of the Malpighian vessels, the serum of the blood gets mixed with the urine, which thus becomes albuminous ; and when the ob struction of the circulation is still greater the colouring matter of the blood escapes from the delicate Malpighian vessels and fills the tubes, giving rise to the haemorrhagic spots before mentioned.
It is only that form of epithelium whose office it is to excrete the solid portion of the urine which becomes gorged with oil ; the delicate epithelium covering the Malpighian vessels, as well as that which lines the straight tubes of the medullary cones, retains its nor mal condition : the reason of these parts re maining healthy while the epithelium of the convoluted tubes becomes greatly changed, as well in cases of fatty degeneration of the kid ney as 'in the desquamative inflammatory diseases before alluded to, will be manifest from a perusal of the second part of this article.
In this form of simple fatty degeneration of the kidney, all the tubes become almost uniformly distended with oil. In a slight degree, and in the earlier stages, it is often found after death in cases where there is no reason to suspect that it has been produc tive of serious mischief during life : it is not until the fatty degeneration exceeds a certain degree that the functions of the organ become seriously affected. It is this form of fatty degeneration which frequently occurs in ani mals, as a consequence of their confinement in a dark room, a fact which was first noticed by Mr. Simon.* The second form of fatty degeneration of the kidney differs from the first in having combined with it more or less of the changes characteristic of desquamative nephritis. The cortical portion of the kidney is soft and pale, and interspersed with numerous small yellow opaque specks. The kidney is generally en larged ; sometimes it is even double the natu ral size. In some cases the cortical portion is somewhat atrophied and granular ; but nei ther in this nor in the first form of fatty degeneration of the kidney does that extreme wasting with granulation occur, which is so frequent a consequence of chronic nephritis.
On a microscopical examination the convo luted tubes are found filled in different degrees with oil, some tubes being quite free, while others are ruptured by the great accu mulation in their interior. The opaque yellow spots scattered throughout the cortical por tion are neither more nor less than convoluted tubes distended, and many of them ruptured by their accumulated fatty contents. The cells which contain the oil are for the most part smaller, more transparent, and less irregular in their outline than the ordinary healthy epithe lium ; they are increased in number, and many of them are so distended with oil as to appear quite black. In parts of the same kidney there may commonly be seen some of the appearances already described as character istic of desquamative nephritis. This form of disease is vefy commonly associated with fatty degeneration of the liver, but less frequently so than the first form of fatty degeneration of the kidney.
The condition of urine connected with this form of renal degeneration is usually, as fol lows : — The quantity is small, the sp. gr. rather above`than below the healthy standard ; it is generally very albuminous, and sometimes bloody. On a microscopical examination of the sediment which is deposited after stand ing for a few hours in a conical glass, there may be seen the fibrinous moulds of the tubes so often alluded to, frequently entan gling blood corpuscles and epithelium. But the main point to be attended to is this, that many of the epithelial cells are naore or less distended with oil. (See figs. 149 and 161.) This fatty condition of the epithelium indi cates with certainty the existence of one of the most serious and intractable diseases to which the kidney is liable. The majority of the cases of acute desquamative nephritis, and many of the chronic cases, end in complete re covery ; but fatty degeneration of the kidney almost invariably leads to general dropsy and a fatal termination. It is therefore as import ant to distinguish between acute or chronic nephritis and fatty degeneration of the kidney as it is to distinguish acute pneumonia or chronic bronchitis from tubercular disease of the lung ; and the diagnosis of the renal disease may be made with as niuch ease and certainty by a microscopical examination of the urine as that of the pulmonary disease by auscultation and percussion of the chest.