Today's Quintilian: 8.6.19-20

XIX. Quod [aliquando] paene iam magis de synecdoche dicam. Nam tralatio permovendis animis plerumque et signandis rebus ac sub oculos subiciendis reperta est: haec variare sermonem potest, ut ex uno pluris intellegamus, parte totum, specie genus, praecedentibus sequentia, vel omnia haec contra, liberior poetis quam oratoribus. XX. Nam prorsa, ut "mucronem" pro gladio et "tectum" pro domo recipiet, ita non "puppem" pro navi nec "abietem" pro tabellis, et rursus, ut pro gladio "ferrum", ita non pro equo "quadrupedem". Maxime autem in orando valebit numerorum illa libertas. Nam et Livius saepe sic dicit: "Romanus proelio victor", cum Romanos vicisse significat, et contra Cicero ad Brutum "populo" inquit "imposuimus et oratores visi sumus", cum de se tantum loqueretur.

19. What I say of metaphor may be applied, perhaps with more force, to synecdoche, for metaphor has been invented for the purpose of exciting the mind, giving a character to things, and setting them before the eye. Synecdoche is adapted to give variety to language by letting us understand the plural from the singular, the whole from a part, a genus from the species, something following from something preceding, and vice versa, but it is more freely allowed to poets than to orators. 20. For prose, though it may admit mucro, "a point" for a sword, and tectum, "a roof" for a house, will not let us say puppis, "a stern" for a ship, or quadrupes, "a quadruped" for a horse. But it is liberty with regard to number that is most admissible in prose. Thus Livy often says, Romanus praelio victor, "The Roman was victorious in the battle," when he means the Romans. Cicero, on the other hand, writes to Brutus, Populo imposuimus et oratores visi sumus, "We have imposed on the people and made ourselves be thought orators," when he speaks only of himself.