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.
) 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.
1. A covering structure of connective tissue, usually of an elongated part, such as the membrane covering a muscle.
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.
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.
A myelin sheath or a neurilemma. See: myelin sheath
The portion of cervical or pretracheal fascia enclosing the carotid artery, interior jugular vein, and vagus nerve.
The fascial covering of femoral vessels.
A fibrous membrane or external investment of the optic nerve.
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
A connective tissue sheath covering a bundle of nerve fibers. Synonym: nerve sheath
An obsolete term for 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 sheathLamellar 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
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.
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.
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 sheathSynovial 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