Serous membranes are often perforated by malignant growths which originated externally to them: the pleura is invaded by masses of cancer deposited in the mediastina, and by large exuberant growths in the mammae; and the peritoneum, amongst other growths, by those which Lobstein has named "Retro-peritoneal." But cancer appears on these membranes as a primary disease also. As a general rule, its appearance there has some connection with the existence of cancer in an organ adjoining, or contained within, the serous sac; so that it always shows the cancerous cachexia to be very far advanced.

The most common forms of cancer found on these membranes are the areolar and the medullary; the latter having not unfrequently the melanotic character. It consists either of laminae, which vary in extent and are unequal in thickness; or of small nodules, like tubercle, which germinate in the tissue of the serous membrane; or of larger knots and tuberous masses, which shoot forth from the tissue over the surface of the membrane. Moreover, upon the serous layer of the dura mater, numerous morbid growths, allied to medullary cancer, are found, the internal and minute construction of which presents very much variety.

In large serous cavities, such as the peritoneum, there are somewhat rarely found very large adventitious growths, which have the same general characters as those under consideration, but are very loosely connected with the serous membrane by one or a few points, or are even entirely unattached.

If, as often happens, inflammation should attack a cancerous growth on a serous membrane, the result is a hemorrhagic exudation: the explanation of the occurrence is found in the very imperfect state of the vascular apparatus involved in the inflammatory process. (Compare what has been said as to the local causes of hemorrhagic exudation).