In the present state of our knowledge it is wiser to put aside the name of Bright's disease and the ideas connected with it, and look for a classification of kidney diseases which will be of practical clini cal use and anatomically correct. There seem to be three ways in which we can classify kidney diseases : according to their causes, ac cording to the part of the kidney involved, or according to the nature of the morbid process.
To classify kidney diseases according to their causes is, in the pres ent state of our knowledge, simply impossible. If, for example, we try to make a class of the kidney diseases caused by scarlet fever, we find that the poison of this disease produces three kidney lesions which differ from each other clinically and anatomically. On the other hand one well-marked form of acute nephritis is caused by scar let fever, by diphtheria, by pregnancy, and occurs without discover able cause. That all forms of nephritis are caused by irritating sub stances in the blood is very probable; that different quantities of the same poison can produce different forms of inflammation has been demonstrated, but we are still very far from being able to construct a classification based on causes.
To classify kidney diseases according to the part of the kidney principally involved is very natural and not at all difficult. There can be no question that disease of the epithelium, of the glomeruli, of the stroma, or of the arteries decidedly predominates in different sets of kidneys. A classification on this basis is anatomically correct. But when we try to use this classification for clinical purposes it does not answer. The history which I have already given of anatomical classifications shows this only too plainly. A classification according to the nature of the morbid process is altogether the most promising. There are three morbid processes which occur in nearly every part of the body, which produce definite anatomical changes, cause regular clinical symptoms, and call for appropriate methods of treatment. These morbid processes are congestion, degeneration, and inflam mation.
Congestion, whether acute or chronic, produces an accumulation of blood in the veins and capillaries of the part affected, causes local symptoms and disturbances of function, and is to be relieved by means addressed to the circulation of the blood.
Degeneration, whether acute or chronic, produces changes more or less profound in the parts affected; is regularly caused by poisons, by disturbances of circulation, and by other diseases ; produces distur bances of function according to its severity; may be itself a cause of inflammation, and can be but little affected by treatment.
Inflammation is attended with three essential features, which may occur separately or together : an escape of the elements of the blood from the vessels, a formation of new tissue, and a death of tissue. So we speak of exudative, productive, and necrotic inflammations.
(a) Exudative inflammation is of short duration, leaves behind it no permanent changes in the parts affected, is sometimes accompa nied by the growth of pathogenic bacteria, and can be favorably affected by treatment.
(b) Productive inflammation runs an acute, subacute, or chronic course. It effects permanent changes in the inflamed parts. Its acute forms are very apt to become chronic. There is much variety as to the relative quantity of exudation and of new tissue. Patho genic micro-organisms may be present.
(c) Necrotic inflammation is characterized by the addition of death of tissue to an inflammation of either exudative or productive type. It is always accompanied by the growth of pathogenic bacteria.
Applying this principle of classification to the kidneys we can dis tinguish : 1. Acute congestion of the kidney.
2. Chronic congestion of the kidney.
3. Acute degeneration of the kidney.
4. Chronic degeneration of the kidney.
5. Acute exudative nephritis.
6. Acute productive nephritis.
7. Chronic nephritis with exudation.
8. Chronic nephritis without exudation.
9. Suppurative nephritis.
10. Tubercular nephritis.