Today's Quintilian: 8.6.14-16

XIV. Vt modicus autem atque oportunus eius usus inlustrat orationem, ita frequens et obscurat et taedio complet, continuus vero in allegorian et aenigmata exit. sunt etiam quaedam et humiles tralationes, ut id de quo modo dixi,"saxea est verruca", et sordidae. XV. Non enim, si Cicero recte "sentinam rei publicae" dixit, foeditatem hominum significans, idcirco probem illud quoque veteris oratoris: "persecuisti rei publicae vomicas". Optimeque Cicero demonstrat cavendum ne sit deformis tralatio, qualis est (nam ipsis eius utar exemplis): "castratam morte Africani rem publicam", et "stercus curiae Glauciam": XVI. ne nimio maior aut, quod saepius accidit, minor, ne dissimilis. Quorum exempla nimium frequenter deprendet qui scierit haec vitia esse. Sed copia quoque modum egressa vitiosa est, praecipue in eadem specie.

14. But as a moderate and judicious use of metaphors adorns language, so a too frequent introduction of them obscures it and renders the perusal of it fatiguing, while a continuous series of them runs into allegory and enigma. Some metaphors, too, are mean, as that which I recently mentioned, "There is a wart of stone, etc." 15. Some are repulsive, for though Cicero uses the _expression sentina rei publicae, "sink of the commonwealth," with great happiness, to signify a herd of bad characters, yet I cannot for that reason approve of the saying of an old orator, Persecuists rei publicae vomicas, "You have lanced the ulcers of the commonwealth." Cicero himself excellently shows that we must take care that a metaphor be not offensive, as in his own examples that "the republic was castrated by the death of Africanus," or that "Glaucia was the excrement of the senate"; 16. that it be not too great, or, as more frequently happens, too little for the subject; and that it be not inapplicable. He who knows that they are faults will find numerous such examples. But an excess of even good metaphors is vicious, especially if they are of the same kind.