from
The Collaborative International Dictionary of English v.0.48
Sheath \Sheath\, n. [OE. schethe, AS. sc[=ae][eth],
sce['a][eth], sc[=e][eth]; akin to OS. sk[=e][eth]ia, D.
scheede, G. scheide, OHG. sceida, Sw. skida, Dan. skede,
Icel. skei[eth]ir, pl., and to E. shed, v.t., originally
meaning, to separate, to part. See {Shed}.]
1. A case for the reception of a sword, hunting knife, or
other long and slender instrument; a scabbard.
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The dead knight's sword out of his sheath he drew.
--Spenser.
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2. Any sheathlike covering, organ, or part. Specifically:
(a) (Bot.) The base of a leaf when sheathing or investing
a stem or branch, as in grasses.
(b) (Zool.) One of the elytra of an insect.
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{Medullary sheath}. (Anat.) See under {Medullary}.
{Primitive sheath}. (Anat.) See {Neurilemma}.
{Sheath knife}, a knife with a fixed blade, carried in a
sheath.
{Sheath of Schwann}. (Anat.) See {Schwann's sheath}.
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from
The Collaborative International Dictionary of English v.0.48
Medullary \Med"ul*la*ry\, a. [L. medullaris, fr. medulla marrow:
cf. F. m['e]dullaire.]
1. (Anat.)
(a) Pertaining to, consisting of, or resembling, marrow or
medulla.
(b) Pertaining to the medula oblongata.
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2. (Bot.) Filled with spongy pith; pithy.
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{Medullary groove} (Anat.), a groove, in the epiblast of the
vertebrate blastoderm, the edges of which unite, making a
tube (the medullary canal) from which the brain and spinal
cord are developed.
{Medullary rays} (Bot.), the rays of cellular tissue seen in
a transverse section of exogenous wood, which pass from
the pith to the bark.
{Medullary sheath} (Anat.), the layer of white semifluid
substance (myelin), between the primitive sheath and axis
cylinder of a medullated nerve fiber.
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