medullary sheath

American  

noun

1. Botany. a narrow zone made up of the innermost layer of woody tissue immediately surrounding the pith in plants.

2. Anatomy. myelin sheath.

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medullary sheath British  

noun

1. anatomy a myelin layer surrounding and insulating certain nerve fibres

2. a layer of thick-walled cells surrounding the pith of the stems of some higher plants

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Etymology

Origin of medullary sheath

First recorded in 1840–50

Example Sentences

The investment of nerve fibers with a protective covering or medullary sheath, consisting of white, fat-like matter.

From Youth: Its Education, Regimen, and Hygiene by Hall, G. Stanley

MS, medullary sheath, the dark, irregular ring just inside the bundles.

From Wood and Forest by Noyes, William

The medullary sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it.

From Physiology and Hygiene for Secondary Schools by Walters, Francis M.

This core is surrounded by the white substance of Schwann, or medullary sheath, which gives the nerve its characteristic ivory-white appearance.

From A Practical Physiology by Blaisdell, Albert F.

It will help in understanding the peripheral system to remember that a nerve consists of a bundle of neurone fibers each wrapped in its medullary sheath and sheath of Schwann.

From The Mind and Its Education by Betts, George Herbert