Axon collateral ( Cajal, 1899 ) : A branch of an axon trunk that is typically thinner than the trunk and arises at a roughly right angle. First pointed out by Deiters (1865, Tab. 1, Fig. 1-b; see Van der Loos, 1967, p. 44), Golgi (1873) established their existence with his black reaction and noted their roughly right angle of origin (he called them branches or side fibrils); apparently named by Cajal (1899, p. 329, "collaterals of axons"). A short form is collateral (Waldeyer, 1891).