THE SEROUS MEMBRANES. 
665 
yet it is a very singular connexion. It is not a continuance of 
the same organization; it is not an interchange of vessels. The 
organ and its membrane, although so closely connected for a 
particular purpose, yet in very many cases, and where we should 
least of all suspect it, have little or no sympathy with each 
other. Pneumonia will sometimes exist, and will run on to 
congestion, or even ulceration, while the pleura will be very little 
affected : and, much oftener, the pleura will be the 3eat of inflam¬ 
mation, it will be attended by increased exhalation to such an 
extent as to suffocate the animal, and yet the parenchymatous 
substance of the lungs will exhibit little other morbid appearance 
than that of mere compression. The disease of a mucous mem¬ 
brane spreads to other parts—that of a serous one is generally 
isolated. 
The Continuity of Texture .—It was to limit the progress of 
disease that this diff erence of structure between the organ and its 
membrane was contrived; and the more to ensure this, the serous 
covering of every viscus, abdominal and thoracic, is perfect. The 
pleura is nowhere perforated even to give passage to the small¬ 
est bloodvessel or nerve, but they enter at one point only, and 
between a duplicature of the membrane. We see the reason of 
this. I have now immediately exposed the lungs to observation— 
a passive organ—a mere mass of cellular texture and innumer¬ 
able vessels—and close by it, or rather between its lobes, I find 
the heart—an active organ, always and energetically at work—a 
mass of muscle, irritable and powerful. Their structure is al¬ 
together different—their function is as different—they have 
nothing in common, except that the blood traverses both, but for 
a totally opposite purpose. They may interfere with and confuse 
the duty of each other,—and the one can scarcely exert an advan¬ 
tageous influence over the other; then observe, although in 
approximation with each other, how perfectly they are separated. 
The double serous investment of the heart, the serous investment 
of the lungs, with the additionally interposed mediastinum, and 
this investment on each, and through its whole extent, impervious, 
render the one organ in a manner independent of the other, and 
the whole is contrived for this especial purpose. We shall see by- 
and-by, that the horse may have or may perish from carditis, and 
the lungs will exhibit no appearance of disease ; and on the other 
hand, he may be destroyed by pneumonia, or by pleurisy, and the 
heart will scarcely sympathize more than any other part of the 
frame. 
The Independence of the Serous Membranes. —Again, do me 
the favour to observe, that between the heart and the mediastinum, 
in the folds of which it is concealed, there is no communication 
