nerve sheath

Also found in: Dictionary, Thesaurus, Encyclopedia.


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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


1. A covering structure of connective tissue, usually of an elongated part, such as the membrane covering a muscle.
2. An instrument introduced into a vessel during angiographic procedures when multiple catheter changes are anticipated. It facilitates ease of change and decreases morbidity at the puncture site.

Patient care

The sheath introduced into the femoral artery, the preferred vascular access route for percutaneous coronary intervention, is a 4 to 6 French (1.35 to 2 mm) in size. The sheath remains in place after completion of the procedure and removal of the catheter until anticoagulation is reversed or anticoagulants are below peak action. The sheath is connected to high-pressure tubing and a flushing system; manual or automatic flushing keeps the line patent. A stopcock connected to the system permits drawing of blood samples.

axon sheath

A myelin sheath or a neurilemma.
See: myelin sheath

carotid sheath

The portion of cervical or pretracheal fascia enclosing the carotid artery, interior jugular vein, and vagus nerve.

crural sheath

The fascial covering of femoral vessels.

dural sheath

A fibrous membrane or external investment of the optic nerve.

femoral sheath

The fascia covering the femoral vessels.

sheath of Henle

See: Henle sheath

sheath of Hertwig

See: Hertwig's root sheath

sheath of Key and Retzius


lamellar sheath

A connective tissue sheath covering a bundle of nerve fibers. Synonym: nerve sheath; perineurium

medullary sheath

An obsolete term for myelin sheath.
Enlarge picture

myelin sheath

Layers of the cell membrane of Schwann cells (peripheral nervous system) or oligodendrocytes (central nervous system) that wrap nerve fibers, providing electrical insulation and increasing the velocity of impulse transmission.
Synonym: Schwann sheath See: nerve fiber; neuron; illustration

nerve sheath

Lamellar sheath.

periarterial lymphoid sheath

The tissue composed of T lymphocytes that surrounds each arteriole in the spleen. The sheaths are attached to lymphoid follicles containing B cells and make up much of the white pulp. See: spleen

pial sheath

An extension of the pia that closely invests the surface of the optic nerve.

sheath of Schweigger-Seidel

The thickened wall of a sheathed artery of the spleen.

rectus sheath

A strong fibrous sleeve in which the rectus abdominis and pyramidalis muscles contract. The sheath is formed from the aponeuroses of the abdominal wall muscles as they meet in the linea alba at the abdominal midline.

root sheath

1. One of the layers of a hair follicle derived from the epidermis. It includes the outer root sheath, which is a continuation of the stratum germinativum, and the inner root sheath, which consists of three layers of cells that closely invest the root of the hair. See: hair
2. The epithelial covering that induces root formation in teeth. Also called Hertwig's root sheath.

synovial sheath

Synovial tendon sheath.

tendon sheath

A dense fibrous sheath that confines a tendon to an osseous groove, converting it into an osteofibrous canal. It is found principally in the wrist and ankle.
See: synovial sheath
Medical Dictionary, © 2009 Farlex and Partners
References in periodicals archive ?
Junji et al examined the MRI features of malignant nerve sheath tumors and concluded that certain MRI features help to distinguish malignant peripheral nerve sheath tumors from neurofibromas.
Abdullah, "Optic nerve sheath diameter measurement: a means of detecting raised intracranial pressure in adult traumatic and non-traumatic neurosurgical patients," American Journal of Emergency Medicine, vol.
Neurofibromas are one of the two types of benign breast peripheral nerve sheath tumours (PNST), the other being breast schwannomas [1, 4, 8, 9].
Usefulness of cytokeratin subsets for distinguishing monophasic synovial sarcoma from malignant peripheral nerve sheath tumor.
Schwannomas are benign nerve sheath tumors that may occur sporadically or in the setting of Neurofibromatosis type 2 (18).
It's still quite weird-looking, but I was told it would take another two years for [the muscles] to settle in fully,' said Basti, who has schwannoma-a benign nerve sheath tumor-near his seventh cranial nerve, which caused paralysis and slacking on the right side of his face.
Schwannomas are benign tumors of peripheral nerve sheath origin, composed of proliferation of Schwann cells in a characteristic pattern.
Peripheral nerve tumors that may manifest in oral cavity includes several differentials such as schwannoma (also known as neurilemmoma or peripheral nerve sheath tumor), neurofibroma, neurinoma in association with multiple endocrine neoplasia, palisaded encapsulated neurinoma, traumatic neuroma and granular cell tumor.2,5 Immunohistochemical reactivity against S-100 and neuron-specific enolase is an essential tool to differentiate these tumors from spindle cell neoplasms of other origin (myofibroblastic tumors, tumors of muscle tissue origin, and fibroblastic tumors).
A 15-year-old boy with neurofibromatosis type 1 (NF1) was referred to us for central venous catheter insertion, and on ultrasound of the neck, he was found to have extensive involvement of the brachial plexus due to the nerve sheath tumour.