Dendritic spine ( Cajal, 1899 ) : A short thin extension from a dendrite (His, 1889) that typically comes off at roughly a right angle. The extension may or may not have a swelling on the end; when it does, the swelling (spine head) is typically postsynaptic. They were discovered and named spines by Cajal (1888, p. 4, "espinas" in Spanish) and named dendritic spines by Cajal (1899-1904, vol. 1, Fig 14); also see Peters et al. (1991, p. 92), Alvarez & Sabatini (2007).