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.]
   [1913 Webster]
   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.
      [1913 Webster]

   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.
         [1913 Webster]

   2. (Zool.) The anterior or cephalic
         ganglion in insects and
      other invertebrates.
     
         [1913 Webster]

   3. The organ or seat of intellect;
         hence, the understanding;
      as, use your brains. " My
         brain is too dull." --Sir W.
      Scott.
      [1913
         Webster]

   Note: In this sense, often used in the
         plural.
         [1913 Webster]

   4. The affections; fancy; imagination.
         [R.] --Shak.
      [1913 Webster]

   5. a very
         intelligent person. [informal]
      [PJC]

   6. the
         controlling electronic mechanism for a robot, guided
     
         missile, computer, or other device exhibiting some degree
 
             of self-regulation. [informal]
      [PJC]

   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]
      [1913 Webster]

   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.
      [1913
         Webster]