334 DISEASES OF THE RESPIRATORY ORGANS. 
bronchial tubes. Under other circumstances the disease is 
centered in the parenchyma of the lung, or extends to the 
pleural membrane; we may, therefore, safely speak of 
affections of the mucous membrane, of the parenchyma, of 
the lung, and of its investing serous membrane. 
Certain general features may be quoted as distinctive of 
each class of affections, and as their consideration will not 
affect the inquiry into the special peculiarities of each 
malady, it may not be uninteresting to introduce them at 
the commencement. 
Diseases of the mucous membrane of the air-passages are 
usually, almost invariably, attended with increased secretion, 
or, more correctly, with exudation from the secreting surface ; 
not necessarily an increase of the normal mucus, but 
rather an additional outpouring of fluids of more or less 
viscidity, rendering the secretion thicker or thinner, and 
otherwise modifying its constitution and appearance. The 
dry stage, so called, during which the secretion is lessened, 
is not of long duration, nor is it often very well marked; 
when properly defined, it is distinguished by a bright colour 
of the membrane and a peculiar deadness of surface, arising 
from the partial drying of the mucus, which is not produced 
in sufficient quantity to preserve the usual moist condition. 
In very many instances a dry cough indicates irritability 
of the laryngeal or bronchial membrane, when there is no 
dryness of the nasal membrane, nor, indeed, an unhealthy 
appearance; probably in such cases, could we inspect the 
part, we should find an absence or diminution of the 
natural secretion. 
Prostration is generally associated with affections of the 
mucous membrane, and that to a much greater extent than 
can be accounted for by the mere loss of fluid from the 
surface; besides being present often too early in the disease 
for the discharge to have produced much effect upon the 
system. In epidemic catarrh this prostration is one of the 
earliest consequences, and forms a very important element 
in the disease. 
The occurrence of a purulent discharge from mucous sur¬ 
faces does not in any great degree complicate the disease 
or its consequences, as a very slight modification of the 
secretive action suffices for the formation of pus in abund¬ 
ance, the difference between the two materials, pus and 
mucus, seeming to exist rather in the fluid menstruum than 
in the characteristic corpuscles, which are closely allied. 
Blood in small quantities may be mixed with the discharge 
without giving grounds for serious apprehension; the rupture 
