These membranes derive their name from the serous or watery fluid they
secrete, by which their surface is constantly moistened. They are to be
found in the three cavities of the chest; namely, one on each side,
containing the right and left lung, and the intermediate cavity,
occupied by the heart. The portion of the membrane lining the lungs is
named the _pleura_, and that lining and covering the heart is called the
_pericardium_. The membrane lining the abdomen is named the
_peritoneum_. The ventricles of the brain are also lined by this
membrane. The serous membranes, after lining their respective cavities,
are extended still farther, by being reflected back upon the organs
enclosed in their cavities; hence, if it were possible to dissect these
membranes from off the parts which they invest, they would have the
appearance of a sac without an opening. In the natural state, these
membranes are exceedingly thin and transparent; but they become
thickened by disease, and lose their transparency. The excessive
discharge of fluids into cavities lined by these membranes constitutes
the different forms of dropsy, on which we shall now treat.