54 
ARRANGEMENT OF SEROUS MEMBRANES. 
Fig. 10. 
Pavement Epithe 
it contains very few nerves. The smooth surface of the mem¬ 
brane forms one unbroken plane, being neither raised into 
villi, nor depressed into follicles; and its basement-membrane 
is covered with a single layer of flat epithelium-cells, which 
are closely applied to it and to each other, 
like the pieces of a pavement (fig. 10). It 
is with such a membrane that every one of 
those great cavities is lined, which contains 
important viscera; and it is also continued 
on to the outer surface of these viscera, 
so as to afford them an external coating* 
over every part save that by which they 
are attached. Thus the heart is suspended 
freely, by the large vessels proceeding from 
lium Cells oe Serous its summit, within a bag or sac of fibrous 
Membrane. membrane peculiar to itself, which is termed 
the pericardium. The cavity of this bag is completely lined 
by the serous membrane (fig. 11, p '), which closely embraces 
the vessels, and which then 
bends down over the surface 
of the heart, so as to enclose 
it in the envelope p. Hence 
it will be seen that this 
membrane, whilst -including 
the heart, and allowing it to 
s communicate with its vessels, 
iP forms a completely shut sac; 
and it may be likened to a 
common double cotton or 
woollen night-cap, which has 
a similar cavity between its 
Fig. H.-DlAGRAM OP THE PERICARDIUM. tW ° ^ S > ^ 
a a , auricles; v v, ventricles ; b, pulmonary really On the Outside 01 
artery; c, aorta; pp f , pericardium. this> wh ilst gee ming to be 
within the envelope. The two layers of the pericardium, 
though separated in the diagram for the sake of distinctness, 
are really in mutual contact, save when separated by the in¬ 
terposition of fluid poured out in disease. Each of the lungs, 
in like manner, is suspended in a closed sac of its own, termed 
the pleura; and the surface of the lung is covered by a serous 
membrane, which is reflected over the wall of the pleural cavity. 
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