Cancer of the pleura is much rarer than tubercle. It never occurs as the first in the series of cancerous deposits occurring in an individual, but is always the result of a cancerous dys-crasia that has previously developed itself in other structures. It is very frequently associated with cancer of the mammary gland, with cancer in the mediastina, with cancer of the osseous system, or with general cancerous disease; it is commonly developed simultaneously with cancer of the lungs, and usually runs a very rapid course after the extirpation of large masses of cancer.

The pleura may be perforated from without by adjacent cancerous formations which, after having established the cancerous metamorphosis on it, sprout forth on its cavity in the form of nodular growths; or the cancer may appear primarily on its inner smooth surface, in the form of flattened, roundish or nodular, lardaceous or medullary masses, varying from the size of a hemp-seed to that of a hen's egg, or even of a man's fist. They are sometimes so fluid as to be diffused and form a stratum, and may be either loosely or firmly attached to the serous membrane on which they are situated. In the latter case they involve the tissues of the pleura itself, and in general turn out to be medullary cancer.

Their presence always occasions an accumulation of more or less serous fluid in the pleural sacs.