Axon varicosity : A swelling along an unmyelinated axon-or unmyelinated segment of an axon (Kölliker, 1896)-that may either form a bouton-of-passage, or may not form a synapse (Foster & Sherrington, 1897) but simply reflect the presence of one or more mitochondria with a diameter greater than the axon. Ehrenberg (1833, p. 453; also see Van der Loos, 1967, Fig. 8) described and named varicose fibers, Cajal (1888, p. 4) described and named varicosities of the neural prolongation [axon] in Golgi preparations of avian cerebellum (Aristotle).