Seldom does an accumulation of water occur in one part of the body without some previous general disorder of the system, or at least an affection of some of the great organs: the heart, liver, or kidneys. We do sometimes meet with " white swelling " of the knee; but nearly always there are also signs of a " scrofulous " constitution to predispose to it.

Inflammation may, however, cause an effusion of serum, which remains after the acuteness of the attack has passed. The simplest illustration of this is seen in a blister.

Suppose mustard to be applied to the skin. First, we see stimulation shown by redness and heat, with very little if any swelling, and no pain. Next, irritation, with soreness and pain, perhaps quite severe; then inflammation, followed by effusion, which raises the skin into what we call a "blister."

So, also, when the pleura, which lines the ribs and wraps the lungs, is inflamed, it throws out in a few days more or less lymph, as an effusion. If this is copious in amount, it presses the lung away, and interferes with its expansion in breathing. This is sometimes so serious a trouble as to induce physicians to tap the chest and draw off the water to relieve the oppressed lung. Likewise, inflammation of the covering of the heart (pericarditis) may result in a serious effusion within the pericardial sac, clogging the heart so as not infrequently to cause death. Hydrocephalus, or water on the brain, may originate in a similar way.

Dropsy of the chest, however, dropsy of the head, dropsy of the abdomen (ascites), and general dropsy, are much more often brought on by obstruction of the circulation, with thinning of the blood, from disease of the liver, kidneys, or heart, or two or more of those organs at the same time. Ovarian dropsy attends a disease of one or both of the ovaries.

Oedema is a watery swelling of a part of the surface of the body or limbs.

Emphysema is a pufiness of the skin, or lungs, from accumulation of air in the cellular substance of the part affected.