Amyloid disease is of more importance in the kidneys than in other organs, both because of itself it leads to albuminuria and because its presence probably induces further structural changes in the organ. These facts induce some authors to include it as a form of Bright's disease. It exists along with amyloid disease in other organs, and is secondary to some disease which affects the body generally, chiefly phthisis pulmonalis and syphilis. In syphilis it not infrequently assumes more of an independent character, and the case, as a whole, may assume the aspect of one of kidney disease.

The view expressed by the author at p. 147 that amyloid disease is produced by the direct action of the morbid poisons concerned in the diseases which lead to it, rather than by the drain on the system which these diseases sometimes cause, is confirmed by the experimental observations of Krawkow, published since the former part of this work was printed. He induced amyloid disease in a number of animals of different kinds by the subcutaneous injection of cultures of staphylococcus pyogenes aureus, but did not succeed in producing the condition when suppurations were produced by other kinds of agents such as turpentine. He expresses the opinion that the products evolved by the microbes constitute the essential cause. He produced amyloid disease in one case by injecting a filtrate of the bacillus pyocyaneus. (See Krawkow, Centralbl. f. allg. Path. u. path. Anat., May, 1895).

The lesion in the kidneys as in other organs affects primarily the walls of the blood-vessels. Even in very early cases, in which the amount of amyloid disease is slight, we usually find it in two distinct and separate structures, namely the Malpighian tufts and the arterire rectse of the pyramids. The Vessels of the glomeruli, generally along with the arteries leading to them, are swelled up and hyaline. The rose colour developed with methylviolet is so striking that in sections stained with this dye it often looks as if the tufts were injected. They are also considerably enlarged, and even without the addition of iodine or methylviolet they form very prominent, transparent, and glancing clumps in the cortex. The Arterise rectae run, as we have seen, in bunches, and their appearance with methylviolet is that of a series of rose-coloured tubes grouped together. They are also sufficiently pronounced as a rule without any reagent, appearing as pearly glancing tubes. While these structures are first and chiefly involved, others usually follow. There is amyloid disease extensively present in the Arteries of the cortex, the ascending and afferent arteries. In these the lesion can often be seen to begin in the muscular coat, a distinct transverse marking being visible either from the muscular fibre cells being mapped out by the staining agent, or by structures lying between the muscle cells being affected. Very often there is here and there a capillary of the cortex affected, and sometimes these are extensively so, even in comparatively slight cases. The Basement membrane of the tubules frequently becomes amyloid in advanced cases, chiefly that of the tubules in the cortical substance (see Fig. 434).

Fig. 434. - Amyloid degeneration of the basement membrane of uriniferous tubules. The thickened and translucent basement membrane is shown with the renal epithelium inside, x 350.

Along with the amyloid disease of the vessels there is usually Fatty degeneration of the epithelium of the tubules, apparently the result of anaemia from the obstruction of the vessels.

Interstitial inflammation nearly always accompanies amyloid disease. It is usually slight in degree, but it may be so great as to produce a combination of chronic interstitial nephritis and amyloid disease, as in Fig. 435, where the glomeruli are brought close together by atrophy of the tubules from interstitial inflammation, and the organ as a whole is contracted.

Fig. 435. - Amyloid and contracted kidney. The highly amyloid and therefore greatly enlarged Malpigbian tufts are shown; their capsules are somewhat thickened. The tufts are very unduly close together, owing to contraction of the intervening tissue. Between them there is round-cell tissue with remains of tubules, x 85.

The mere existence of amyloid disease probably gives rise to interstitial inflammation, but it is not improbable that the primary disease which led to the amyloid condition may also induce nephritis. Thus in syphilis the virus affecting the kidney directly may cause interstitial inflammation, while less directly it produces a general amyloid condition. Again, in phthisis pulmonalis, and in extensive suppurations from diseased bone, there may readily occur an absorption of irritating materials which are capable of setting up inflammation in process of excretion by the kidneys.

The contracted amyloid kidney is not generally accompanied by hypertrophy of the heart, the reason apparently being that, as the blood is deteriorated, the compensatory growth of muscular tissue can scarcely occur.

In regard to the Naked-eye appearances, the simple amyloid kidney is enlarged, sometimes greatly enlarged, so that the organ weighs nine or ten ounces. The surface is pale and the capsule comes off easily. On section the cortex is seen to be thickened and pale in colour. The general appearance therefore resembles that of the large white kidney, and doubtless, many cases have been mistaken for that. But the organ in amyloid disease has a firm elastic feeling and a transparent bacony appearance on section, which is distinctive enough. The thick pale transparent cortex usually contrasts with the redder pyramidal substance, and the appearance has been somewhat aptly compared to the section of bacon ham. The name Lardaceous kidney is often applied from the transparent appearance of the tissue. In the midst of the transparent basis opaque yellow streaks are often visible, from the fatty degeneration of the epithelium. The addition of a solution of iodine to the cut surface of such a kidney brings out the affected glomeruli as brown dots and the arterise rectse as brown streaks. If the kidney be contracted, the granulations on the surface show the usual transparent waxy appearance of the amyloid kidney, and on section the cortex, though greatly thinned, shows a similar condition.

The amyloid substance apparently allows fluid to pass through it more readily than ordinary albuminous structures. Injection of the amyloid kidney shows that the vessels of the glomeruli and the arteries of the cortex are largely obstructed. It may well be that this leads to increased blood-pressure in the remaining arteries, but with the general narrowing of the smaller arteries this can hardly tell with much force on the glomeruli which remain pervious. This excessive secretion of urine, like the diarrhoea which is a prominent symptom in amyloid disease of the intestine, is probably to be accounted for by a greatly increased permeability of the vessels.

2. Atrophy

A local atrophy sometimes occurs from Endarteritis (see Fig. 436). There are flat depressions on the surface which may give the kidney a somewhat coarsely granular appearance, but sometimes these are extensive, involving considerable areas of the kidney. In this latter case the larger arteries running between cortex and pyramids are affected with endarteritis, which may be an Atheroma associated with a similar condition in other arteries. The thickening of the internal coat is very extreme (see below e in Fig. 436), and in the smaller branches amounts to absolute occlusion.

The atrophy has considerable resemblance in its details to that in chronic interstitial nephritis, but also presents considerable differences. It is not limited to the cortex, but involves the pyramids as well, and there is much less distortion but more of a simple diminution or disappearance of the tubules. The affected areas are very strikingly reduced in volume, so that the hilum of the kidney comes close to the surface (see between a and e in figure). This contrasts greatly with the neighbouring unaffected parts in which the cortex is even more voluminous than normal.

By the Atrophy of the tubules the glomeruli are brought close together, so close in many cases as to show that the tubules have virtually disappeared. In the pyramidal portion the atrophy of the tubules causes the intertubular substance to become very prominent as a clear translucent matrix interrupted by small round areas. The Glomeruli are themselves in a state of sclerosis. At the marginal parts of the affected areas this condition is seen to begin by a thickening of the capsule which is less fibrous than in the sclerosis of interstitial nephritis, baft has rather a hyaline appearance. The occlusion of the vessels in the tuft and the solidification of the glomerulus follow. In many parts the remaining tissue consists of little more than closely aggregated and solidified glomeruli, these being, apparently, in their altered state, very persistent.

Fig. 430. - Local atnophy from endarteritis, a, hilum of kidney; B, normal pyramid; c, normal cortex showing glomeruli, medullary rays, etc.; d, atrophy beginning, the convoluted tubules atrophied while glomeruli remain. On approaching C the glomeruli become greatly contracted and close together, x 6.

In addition to these evidences of atrophy there is some infiltration of round cells, but not any considerable new-formation of fibrous tissue.

The description given above of local atrophy from endarteritis is based on the observation of two well-marked cases in which nearly half the kidney tissue was atrophied, the locality of the atrophy being chiefly in the neighbourhood of the hilum, and generally continuous, although also with isolated patches. An interesting feature in both these cases was the co-existence of hypertrophy of the left ventricle. The importance of the fact that local atrophy of the kidneys may have such a result has already been referred to.

In Senile atrophy the whole structures of the kidneys undergo diminution in size, but very often this affects especially the secreting epithelium of the convoluted tubules. If this be the case there maybe a special shrinking of the cortex, and the kidneys may be like those of chronic Bright's disease. Sometimes also the surface is granular and the capsule adherent.