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a tubular case or envelope.
arachnoid sheath the delicate membrane between the pial sheath and the dural sheath of the optic nerve.
carotid sheath a portion of the cervical fascia enclosing the carotid artery, internal jugular vein, vagus nerve, and sympathetic nerves supplying the head.
connective tissue sheath of Key and Retzius endoneurium.
crural sheath femoral sheath.
dural sheath the external investment of the optic nerve.
femoral sheath the fascial sheath of the femoral vessels.
Henle's sheath endoneurium.
lamellar sheath the perineurium.
medullary sheath myelin sheath.
myelin sheath (nerve sheath) the sheath surrounding the axon of myelinated nerve cells, consisting of concentric layers of myelin formed in the peripheral nervous system by the plasma membrane of Schwann cells, and in the central nervous system by the plasma membrane of oligodendrocytes. It is interrupted at intervals along the length of the axon by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerve impulses (see saltatory conduction).
pial sheath the innermost of the three sheaths of the optic nerve.
root sheath the epidermic layer of a hair follicle.
sheath of Schwann neurilemma.
synovial sheath synovial membrane lining the cavity of a bone through which a tendon moves.
tendon sheath a lubricated fibrous or synovial layer of tissue in which the tendon is housed and through which it moves.
the lipoproteinaceous envelope in vertebrates surrounding most axons larger than 0.5-mcm diameter; it consists of a double plasma membrane wound tightly around the axon in a variable number of turns and supplied by oligodendroglia cells (in the brain and spinal cord) or Schwann cells (in peripheral nerves); unwound, the double membrane would appear as a sheetlike cell expansion devoid of cytoplasm but for a few narrow cytoplasmic strands corresponding to apparent interruptions of the regular myelin structure, the incisures of Schmidt-Lanterman. The myelin sheath of each axon is composed of a fairly regular longitudinal sequence of segments, each corresponding to the length of sheath supplied by a single oligodendroglia or Schwann cell; in the short interval between each two neighboring segments, the nodes of Ranvier, the axon is unmyelinated even though it is enclosed by complex, fingerlike, membranous expansions of the neighboring oligodendroglia or Schwann cells.
Synonym(s): medullary sheath
See myelin sheath.
An obsolete term for myelin sheath.
See also: sheath