Membranes are thin layers of epithelial tissue usually bound to an
underlying layer of connective tissue. Membranes cover, protect, or separate
other structures or tissues in the body. The four types of membranes are: 1)
cutaneous membranes; 2) serous membranes; 3) mucous membranes; and 4) synovial
membranes. The cutaneous membrane is skin. Skin consists of a layer of
stratified squamous epithelium (epidermis) firmly attached to a thick layer of
dense connective tissue (dermis). It differs from other membranes because it is
exposed to air and is dry. The serous membranes (or serosae) consist of simple
squamous epithelium (a mesothelium) supported by a layer of connective tissue
(areolar). These moist membranes line the closed, internal divisions of the
ventral body cavity. The three types of serous membranes are: 1) the pleura,
lining the pleural cavities and covering the lungs; 2) the peritoneum, lining
the peritoneal cavity and covering the abdominal organs; and 3) the
pericardium, lining the pericardial cavity and covering the heart. The mucous
membranes (or mucosae) consist of epithelial tissue (usually stratified
squamous or simple columnar epithelia) on a layer of loose connective tissue
called the lamina propria (from the Latin, meaning “one’s own layer”). The
mucosae line the body cavities that open to the exterior, such as the
digestive, respiratory, reproductive, and urinary tracts. These membranes are
kept moist by bodily secretions. Synovial membranes are composed of connective
tissue. They surround the cavity of joints, filling the space with the synovial
fluid that they make. The synovial fluid lubricates the ends of the bones
allowing them to move freely.
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