Dropsy consists in the accumulation of lymph, either in small lymph spaces in the tissues (oedema, anasarca) or large serous cavities (ascites, pleural or pericardial effusions). The accumulation is caused by more lymph being poured out from the capillaries than can be removed by the lymphatics and veins,

The chief causes of dropsy are - (1) Diminished removal of lymph from the lymph spaces or serous cavities. This may be due to (a) obstruction of the veins, or (b) of the lymphatics. (2) Increased exudation of lymph from the capillaries. This increased exudation may be due to (a) changes in the walls of the capillaries themselves rendering them more permeable. This appears to be the only condition which by itself can produce oedema. There are two others, however, which, although by themselves incapable of producing oedema, yet, along with others, are of the utmost importance; these are (b) a watery condition of the blood, and (c) vaso-motor paralysis. In many, indeed in most cases of dropsy, two or three of these factors are combined.

Fig. 119.   Diagram of Ranvier's experiment on dropsy. The vena cava is ligatured, and in the left leg the trunk of the sciatic has been divided so that both the motor and vaso motor nerves contained in it are paralysed.

Fig. 119. - Diagram of Ranvier's experiment on dropsy. The vena cava is ligatured, and in the left leg the trunk of the sciatic has been divided so that both the motor and vaso-motor nerves contained in it are paralysed. On the right side the motor roots of the sciatic alone are divided and the vaso-motor left uninjured. There is thus motor paralysis on both sides, but vaso-motor paralysis and dropsy only on the left side.

Obstruction to the veins, or lymphatics alone, will rarely cause dropsy, unless at the same time there is increased transudation from the capillaries. Thus Ranvier found that ligaturing the vena cava of a dog did not produce dropsy in the legs, the lymph being removed either by the collateral venous circulation or by the lymphatics. On dividing the sciatic nerve on one side, however, after ligature of the vena cava, dropsy appeared in the corresponding leg, while it remained absent from the other. He showed that the dropsy was caused by paralysis of the vasomotor, and not of the motor fibres contained in the sciatic, by dividing the motor roots of the sciatic on the other side, leaving the vaso-motor roots uninjured. When this was done motor paralysis occurred equally in both legs, but dropsy only appeared in the one where the vaso-motor nerves had been divided (Fig. 119). This experiment shows what an important factor the loss of vascular tone is in the production of oedema, and we may legitimately infer from it that vascular tonics, by increasing the contractility of the vessels, will tend to prevent oedema, or remove it when it is already present.

A watery condition of the blood does not of itself increase the exudation of lymph, nor does it produce oedema, yet in cases of anaemia or chlorosis we very frequently find a tendency to oedema of the ankles, and experiments in Cohnheim's laboratory have shown that, although a watery condition of the blood alone causes no increased exudation of lymph so long as the vaso-motor nerves are intact, yet it does so to a very great extent when the vasomotor nerves are paralysed.1

Alteration of the capillaries by inflammation causes increased exudation of lymph, and tends to produce a local oedema. This oedema is greatly increased if the vaso-motor nerves are paralysed, not only attaining a much greater extent, but appearing more quickly and lasting longer. I have already mentioned that, in experiments on artificial circulation, acids added to the circulating fluid not only caused dilatation of the vessels, but increased transudation through them, and tended to render the tissues oedematous. It is not improbable that some alterations of the blood-vessels of the living body which tend to render them more permeable may be connected with imperfect oxidation and the formation of sarco-lactic instead of carbonic acid.

Arsenic has this power of lessening oxidation,2 and it seems not improbable that the tendency to produce oedema of the eyelids which it possesses may be due to this peculiar action.

1 Jankowski, Virchow's Archiv, xciii. p. 259.

2 Feitelberg, Inaug. Diss. Dorpat, 1883.

It is evident that whatever tends to increase oxidation will have an opposite effect, and will tend to prevent any excessive exudation from the capillaries. In cases of anaemia iron is therefore serviceable, and as the condition of the blood improves the tendency to oedema disappears.

What has just been said regarding the action of acids may seem to be in contradiction to the usually received opinion that the mineral acids act as vascular tonics. It is quite true that small doses of dilute acids, especially when given, as they usually are, along with bitters, frequently impart a feeling of strength and tone, whereas alkalies are frequently felt to be depressing, but in the case of both these classes of remedies this effect is probably not due to any direct action on the vessels themselves (vide Acids).