sheath
[shēth] 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.
dural sheath the external investment of the optic nerve.
femoral sheath the fascial sheath of the femoral vessels.
lamellar sheath the perineurium.
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.
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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.