reas a
pleural or peritoneal effusion is always highly albuminous. Also an
effusion due to heart disease contains more albumin than one due to
kidney disease. In appearance it may be colourless, greenish or reddish
from the presence of blood pigment, or yellowish from the presence of
bile pigment; transparent or opalescent or milky from the presence of
fatty matter derived from the chyle. The membrane from which the
dropsical fluid escapes is healthy, or at least not inflamed, and only
somewhat sodden by long contact with the fluid--the morbid condition on
which the transudation depends lying elsewhere.

The simplest cause of dropsy is purely mechanical, blood pressure being
raised beyond a certain point owing to venous obstruction. This may be
due to thrombosis of a vein as in phlegmasia dolens (white leg),
retardation of venous circulation as in varicose veins, or obstruction
of a vein due to the pressure of an aneurism or tumour. Cardiac and
renal dropsy are more complicated in origin, but cardiac dropsy is
probably due to diminished absorption, and renal dropsy, when
unassociated with heart failure, to increased exudation. But the
starting point of acute renal dropsy, of the dropsy sometimes occurring
in diabetes, and that of chlorosis is the toxic condition of the blood.
For accounts of the various local dropsies see HYDROCEPHALUS; ASCITES;
LIVER, &c.; general dropsy, or dropsy which depends on causes acting on
the system at large, is due chiefly to diseases of the heart, kidneys or
lungs, occasionally on lardaceous disease, more rarely still on diabetes
or one of the anaemias.

Broadly speaking, 50% of cases of general dropsy are due to disease of
the heart or aorta, and 25% to renal troubles. The natural tendency of
all diseases of the heart is to transfer the blood pressure from the
arteries to the veins, and, so soon as this has reached a sufficient
degree, dropsy in the form of local _oedema_ commences to appear at
whatever may be the most depending part of the body--the instep and
ankle in the upright position, the lower part of the back or the lungs
if the patient be in bed--and this tends gradually to increase till all
the cavities of the body are invaded by the serous accumulation. The
diseases of the lungs which produce dropsy are those which obstruct the
passage of the blood through them, such as emphysema and fibrosis, and
thus act precisely like disease of the heart in transferring the blood
pressure