Hi Listers,
Here is part 2.
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oozing on the muggy shore of the gulf coast
[email protected]
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Greetings elegant and lovely partakers of the CS art,
MEMBRANE
The word membrane in its widest sense is used to designate any thin
expansion of tissues. In the commonest sense, the word *membrane* is
used to denote an enveloping or a lining layer made up of tissues.
The chief membranes of the body are classified as serous, synovial,
mucous and cutaneous. We will consider the mucous membrane.
MUCOUS membranes may be grouped in two great divisions: gastropulmonary
and genitourinary. Mucous membranes secrete *mucus*, a watery fluid
containing *mucin*( a glycoprotein),salts,etc.
The *gastropulmonary* mucous membrane lines the alimentary canal, the
air passages, and the cavities communicating with them. It commences at
the edges of the lips and nostrils,extends through the mouth and nose to
the throat, and is continued throughout the length of the alimentary
canal to the anus. At its origin and termination it is continuous with
the external skin. It also extends throughout the trachea, bronchial
tubes and air sacs (lungs). From the interior of the nose the membrane
extends into the frontal, ethmoidal, sphenoidal and maxillary sinuses,
also into the lacrimal passages, becoming the conjunctival membrane over
the fore part of the eyeball and inside of the eyelids, on the edges of
which it meets the skin.
A prolongation of this membrane extends on each side of the pharynx,
forming the lining of the auditory (eustachian) tube. This membrane also
lines the salivary, pancreatic, and biliary ducts, and the gall bladder.
STRUCTURE
A mucous membrane is usually composed of four layers of tissue:
epithelium, basement membrane, areolar connective tissue, and the
muscularis mucosae.
Epithelium is the surface layer. In the stomach and intestine it is
columnar as shown previously. It is devoid of blood capillaries and fed
by the lymph.
The basement membrane, the second layer, consists of a layer of
flattened cells, the differentiated edge of the underlying connective
tissue or a secretion of the epithelial cells. It is nourished by the
lymph.
The stroma (corium) is composed of either areolar or lymphoid
connective tissue and contains blood capillaries.
The forth layer is the muscularis mucosae which consists of a thin
layer of muscular tissue.
The blood is never allowed to come in contact with the first two
layers. The blood tissue is to fragile and easily contaminated for the
body to permit it to come into contact with the outside environment.
Only the lymph is permitted to approach the outside world. More on this
later.
The mucous membranes are attached to the parts beneath them by areolar
connective tissue, here called *submucous* connective tissue. This
differs greatly in quantity as well as in consistency in different parts
of the body. The connection is in some cases close and firm, as in the
cavity of the nose. In other instances, especially in cavities subject
to frequent variations in capacity, like in the esophagus and the
stomach, it is lax. When such a cavity is narrowed by contraction of its
outer coats, the mucous membrane is thrown into folds, or *rugae*, which
disappear again when the cavity is distended. In certain parts the
mucous membrane forms permanent folds that cannot be effaced, and these
project conspicuously into the cavity which it lines. The best example
of these folds is seen in the small intestine, where they are called
*circular folds* (valvulae conniventes), which increase the area of
absorbing surface for the products of digestion. In some locations the
free surface of mucous membrane contains minute glands (goblets) or is
covered with papillae or villi.
Functions of the mucous membranes are protection,support of blood
vessels and lymphatics, and provision of a large amount of surface for
secretion and absorption.
A mucous membrane protects by forming a lining for all the passages
that communicate with the exterior,i.e., those passages subject to the
contact of foreign substances which are introduced into the body and of
waste materials which are expelled from the body. The mucus secreted is
a thicker and more viscid liquid than either serum or synovia and, by
coating the surface, lessens the irritation from food, waste material,
or secreted substances.
The redness of mucous membranes is due to their abundant supply of
blood capillaries in the stroma layer (corium) forming a vast network
just under the basement membrane. The lymphatics also form networks in
the corium and communicate with larger vessels in the submucous tissue
below.
The projections of mucous membrane, such as the circular folds, covered
with glands and villi as they are, increase enormously the surfacs area
of the membrane for secretion and absorption and enable the membrane to
carry more blood vessels and lymphatics.
Bless you Bob Lee
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oozing on the muggy shore of the gulf coast
[email protected]
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