FALSE MEMBRANES. 497 



IV. FALSE MEMBRANES. 



This expression is a very objectionable one; since it may either 

 mean merely a coagulated exudation, spread upon a surface, like a 

 membrane, or the same exudation after it has become organized. E. 

 g. the merely fibrillated and never vascular exudation of croup, is 

 termed a false membrane, as well as the highly vascular membrani- 

 form formation, so common on the surface of the pleura in conse- 

 quence of inflammation of this membrane. The former should be 

 called merely a coagulated exudation; while the latter may be termed 

 a, false membrane, if we intend by this expression to indicate the fact 

 shown t by the microscope, that these formations are not (histologi- 

 cally) membranes, though they sometimes, from their form, appear 

 to the unaided eye to be such. 



False membranes are, therefore (if the term is to be retained), 

 more or less organized exudations of plasma, and are developed espe- 

 cially upon serous membranes (p. 188, 2). They, however, present 

 all grades of development and vascular ity, according to the time 

 elapsing since the occurrence of exudation. When fully formed, 

 they consist of a layer of imperfectly developed areolar tissue, con- 

 taining a vascular network, and sometimes even nerves also, and 

 lymphatics. Consequently, they are really mere new formations of 

 areolar tissue. 



False membranes being a new formation are prone to involution, 

 and ultimately may entirely disappear. Fatty degeneration in them 

 is very common. They are also frequently the seat of pathological 

 epigeneses, especially of tubercle. 



The bands and adhesions so frequently resulting from pleuritis 

 and peritonitis, are histologically identical with false membranes; 

 being also new formations of areolar tissue. 



New membranes are sometimes formed to cover the surface of per- 

 manent adventitious cavities; e.g. the membrane lining cavities in 

 the lung, formed by the removal of a mass of tuberculous depo- 

 sit, &c. 



A new formation of epithelium occurs in many pathological cysts; 

 of the internal surface of which the new formation constitutes the 

 lining. 



32 



