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.

---

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

---

Etymology

Origin of medullary sheath

First recorded in 1840–50

Example Sentences

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

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

From Wood and Forest by Noyes, William

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.

The medullary sheath may also aid in the nourishment of the axis cylinder.

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

In the white masses the medullary sheath is present.

From The Mind and Its Education by Betts, George Herbert