medullary sheath

sheath (shēth) a tubular case or envelope.

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arachnoid sheath  the continuation of the arachnoidea mater around the optic nerve, forming part of its internal sheath.

carotid sheath  a portion of the cervical fascia enclosing the carotid artery, the internal jugular vein, and the vagus nerve.

crural sheath  femoral s.

dentinal sheath  the layer of tissue forming the wall of a dentinal tubule.

dural sheath  the external investment of the optic nerve.

femoral sheath  the investing fascia of the proximal portion of the femoral vessels.

sheath of Henle  endoneurium.

Hertwig sheath  root s. (1).

sheath of Key and Retzius  endoneurium.

lamellar sheath  the perineurium.

Mauthner's sheath  axolemma.

medullary sheath , myelin 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 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.

pial sheath  the continuation of the pia mater around the optic nerve, forming part of its internal sheath.

root sheath 

1. an investment of epithelial cells around the unerupted tooth and inside the dental follicle.

2. the epithelial portion 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  epitendineum.

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medullary sheath

n.

See myelin sheath.