one of the parts of a speech. But the _Ad Herennium_ does make _divisio_ a
part of a speech,[160] and does require not over three parts.[161] As late
as 1612, Thomas Heywood quotes the authority of "Tully, in his booke _Ad
Caium Herennium_."[162]

The relative importance of Cicero's rhetorical works to the middle ages is
well illustrated by a count of the manuscripts preserved. In the libraries
of Europe today there exist seventy-nine manuscripts of the _De
inventione_, eighty-three of the _Ad Herennium_, forty of the _De
oratore_, fourteen of the _Brutus_, and twenty of the _Orator._[163] Thus
in the University of Bologna the study of rhetoric was based on the _De
inventione_ and the _Ad Herennium_.[164] The _De inventione_ is the source
for Alcuin's rhetorical writings, and was the only Ciceronian rhetoric
known to Abelard or Dante. Brunette Latini translated seventeen chapters
of it into Italian.[165] Although mutilated codices of the _De oratore_
and the _Orator_ were known to Servatus Lupus and John of Salisbury,
complete manuscripts of these most important works were not known previous
to 1422.[166] The _Ad Herennium_ and the _De inventione_ were first
printed by Jenson at Venice in 1470. The first book printed at Angers
(1476) was the _Ad Herennium_ under the usual mediaeval title of the
_Rhetorica nova_. The first edition of the _De oratore_ was printed in the
monastery of Subaco about 1466. The _Brutus_ first appeared in Rome (1469)
in the same year which witnessed the first edition of the Orator.[167]
Before its first printing the _Orator_ was used as a reference book for
advanced students by Guarino in his school at Ferrara.

Castiglione's indebtedness to the _De oratore_ is well known, but few
notice that his first paragraphs are a close paraphrase of Cicero's
dedicatory paragraphs of the _Orator._