anacoluthon (n.)

"want of grammatical sequence; changing constructions in mid-clause," whether arbitrary or intentional, 1706, from Latinized form of Greek anakoluthon, neuter of anakolouthos "inconsequent," from an- "not" (see an- (1)) + akolouthos "following," from copulative prefix a- expressing union or likeness (see a- (3)) + keleuthos "way, road, track, path, course, journey," which is of unknown etymology. "As a figure of speech it has propriety and force only so far as it suggests that the emotion of the speaker is so great as to make him forget how he began his sentence" [Century Dictionary]. Related: Anacoluthic.

Anacoluthon, though a grammatical defect, is a rhetorical beauty, if naturally produced or imitated; as, "If thou art he—but oh ! how fallen!" "He who hath seen life in all its shapes, and fully knows its good and evil—No ! there is nothing on earth which can make a wise man desire a greater length of days than heaven appoints." These are instances in which the break down is the effect of emotion. [James R. Boyd, "Elements of English Composition," 1874]