The Collaborative International Dictionary of English v.0.48:
Brain \Brain\ (br[=a]n), n. [OE. brain, brein, AS. bragen,
br[ae]gen; akin to LG. br[aum]gen, bregen, D. brein, and
perh. to Gr. bre`gma, brechmo`s, the upper part of head, if
[beta] = [phi]. [root]95.]
1. (Anat.) The whitish mass of soft matter (the center of the
nervous system, and the seat of consciousness and
volition) which is inclosed in the cartilaginous or bony
cranium of vertebrate animals. It is simply the anterior
termination of the spinal cord, and is developed from
three embryonic vesicles, whose cavities are connected
with the central canal of the cord; the cavities of the
vesicles become the central cavities, or ventricles, and
the walls thicken unequally and become the three segments,
the fore-, mid-, and hind-brain.
Note: In the brain of man the cerebral lobes, or largest part
of the forebrain, are enormously developed so as to
overhang the cerebellum, the great lobe of the
hindbrain, and completely cover the lobes of the
midbrain. The surface of the cerebrum is divided into
irregular ridges, or convolutions, separated by grooves
(the so-called fissures and sulci), and the two
hemispheres are connected at the bottom of the
longitudinal fissure by a great transverse band of
nervous matter, the corpus callosum, while the two
halves of the cerebellum are connected on the under
side of the brain by the bridge, or pons Varolii.
2. (Zool.) The anterior or cephalic ganglion in insects and
3. The organ or seat of intellect; hence, the understanding;
as, use your brains. " My brain is too dull." --Sir W.
Note: In this sense, often used in the plural.
4. The affections; fancy; imagination. [R.] --Shak.
5. a very intelligent person. [informal]
6. the controlling electronic mechanism for a robot, guided
missile, computer, or other device exhibiting some degree
of self-regulation. [informal]
To have on the brain, to have constantly in one's thoughts,
as a sort of monomania. [Low]
no-brainer a decision requiring little or no thought; an
obvious choice. [slang]
Brain box or Brain case, the bony or cartilaginous case
inclosing the brain.
Brain coral, Brain stone coral (Zool.), a massive
reef-building coral having the surface covered by ridges
separated by furrows so as to resemble somewhat the
surface of the brain, esp. such corals of the genera
M[ae]andrina and Diploria.
Brain fag (Med.), brain weariness. See Cerebropathy.
Brain fever (Med.), fever in which the brain is specially
affected; any acute cerebral affection attended by fever.
Brain sand, calcareous matter found in the pineal gland.