40 PRIMARY TISSUES :—SEROUS MEMBRANES. . 
and has sometimes been purposely effected, does not produce 
any disorder in the general functions of the body. In blow- 
ing the nose violently, some part of the membrane lining its 
cavity has occasionally given way, so as to allow air to 
into the areolar tissue of the face, and especially into that 
contained in the eyelids, which is particularly loose ; an enor- 
mous swelling of these parts then takes place, presenting a 
very frightful appearance, but not attended with the least 
danger, and subsiding of itself in a few days. This swelling 
presents a character to the touch quite different from that 
which would be occasioned by a similar distension with liquid; 
for it gives somewhat of the crackling feel that is occasioned 
by pressing on a blown bladder. A similar inflation of the 
areolar tissue of the body has sometimes occurred from the 
formation of an aperture, by disease or injury, in the walls of 
the lungs or air-passages, and the consequent escape of air 
during the act of breathing: in one remarkable case of this 
kind, the skin of the whole body was so tightly distended 
with air as to resemble a drum. It is intentionally practised 
‘ by butchers, who “blow up” the areolar tissue of their veal, 
in order to increase its plumpness of aspect; and the in- 
flation of the areolar tissue of the head, in the living state, 
has been sometimes practised by impostors, in order to excite 
commiseration. 
28. Fibres and shreds of fibro-membrane, resembling those 
of which areolar tissue is composed, may be so interwoven as 
to form a continuous sheet of membrane, having a smooth 
and glistening surface ; and in this manner are produced the 
Serous Membranes that line the different cavities in which the 
viscera (or organs contained within the skull, the chest, and 
the abdomen) are lodged. The peculiar manner in which 
these membranes are arranged, will be explained hereafter 
(§ 43). One of their surfaces is always free or unattached, 
whilst the other is in contact with the outer wall of the 
cavity; and from the free surface, which is covered with a 
layer of flattened epithelium-cells (fig. 10), a serous fluid is 
exhaled, which adds to its smoothness. It is by an accumula- 
tion of this fluid, that dropsies of the cavities are produced,— 
such as water on the brain, or in the chest. 
29. By the union of fibres of a stronger kind, those firmer 
tissues are produced, which are employed wherever a greater 
