The Collaborative International Dictionary of English v.0.48:

Protagon \Pro"ta*gon\, n. [Proto- + Gr. 'agw`n 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.
   [1913 Webster]