The rest of _Orator_, about one third (l-lxxi, 168-236) is devoted to prose rhythm under four heads: (a) origin, (b) cause, (c) nature, (d) use. Under the first Cicero develops a rhetorical doctrine of rhythm from Thrasymachus, Isocrates, and Gorgias. As to the second, its cause (177-8), he says: “The ear, or the mind through the ear, contains in itself a certain natural measure[92] of all spoken sounds.” The third heading, the nature of rhythm, is treated at greater length (liii-lx, 179-203). Analyzing rhythm to show that it has an effect distinct from that of mere euphony, Cicero goes on to examine what this effect is (183). Since there is a distinct rhythmical effect in prose, it can be explained, though it appeals to sense, not to reason, and though it is less obvious and less essential than in verse. It is lacking (186) in earlier writers, Herodotus for example.[93] It has to be sought as a final grace of prose (186). “If there is (187) prose stinted and concise and other prose dilated and fluent, the difference must arise not from the nature of letters (_litterarum_), but from the variety of intervals, long and short; and since prose is now steady, now shifting, according as it is woven and blended with these intervals, the nature of the difference (or of this variety) must reside in the rhythm (_numeris_, 187).”[94] Prose being unmetrical, however, are its rhythms (188) still the same as those of verse? The feet must be the same; but what rhythms are available in prose? That any foot is possible appears in that we often fall carelessly into meter. Prose consists largely of iambs; but we often lapse into less familiar meters. It is plain, then, that prose feet are the same as poetic.