Source : Webster's Revised Unabridged Dictionary (1913)

Protagon \Pro"ta*gon\, n. [Proto- + Gr. ? a contest. See.
   {Protagonist}. So called because it was the first definitely
   ascertained principle of the brain.] (Physiol. Chem.)
   A nitrogenous phosphorized principle found in brain tissue.
   By decomposition it yields neurine, fatty acids, and other
   bodies.