THE 
VETERINARIAN. 
vol. v. DECEMBER, 1832. No. go. 
MR. YOUATT’S VETERINARY LECTURES, 
DELIVERED AT THE UNIVERSITY OF LONDON. 
LECTURE XXIII. 
The Serous Membranes .— The Pleura . 
The parietes of the thorax are lined, and the lungs are cover¬ 
ed, by a smooth glistening membrane, the pleura. It is a serous 
membrane, so called from the nature of its exhalation,— serous — 
albuminous—in distinction from the mucous secretion yielded by 
the membrane of the air-passages that have hitherto come under 
our consideration. The serous membrane generally invests the 
most important organs, and always those that are essentially 
connected with life ; while the mucous membrane lines the interior 
of the greater part of them. The pleura is the investing mem¬ 
brane of the lungs, and a mucous membrane the lining one of 
the bronchial tubes. 
The Polish of the free Surface. —Among the principal cir¬ 
cumstances to be observed with regard to the pleura is the polish 
of its external, or, as it may perhaps be better termed, free 
surface. The glistening appearance of the lungs, and of the in¬ 
side of the costal wall, is to be attributed entirely to the mem¬ 
brane by which they are covered ; for if we strip off the pleura, 
the surface of the lungs and of the intercostal muscles is not a 
little rough. This is the character by which the serous membrane 
is distinguished everywhere, and thus the motion cf the various 
organs is freer and less dangerous. Although the lungs and the 
thorax are in constant approximation with each other, both in 
expiration and inspiration, yet in the frequently hurried and 
violent motion of the animal, and in fact in every act of expiration 
and inspiration, of dilatation and contraction, much and injurious 
friction would ensue if the surfaces did not glide freely over each 
other, from the peculiar polish of this membrane. 
We shall have to observe, by-and-by, how admirably the 
serous membranes prevent injurious friction, even in disease as 
well as in health. In health the polished surfaces glide smoothly 
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