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

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

From Project Gutenberg

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 Project Gutenberg

Surrounding the axis cylinder is a thick, whitish-looking layer, known as the medullary sheath, and around this is a thin covering, called the primitive sheath, or neurilemma.

From Project Gutenberg

Each one of the perforations is the entrance to an oblique tunnel, which is bored in the medullary sheath of the twig.

From Project Gutenberg

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

From Project Gutenberg