For more on Demetrius and our words for period, comma, and colon, see this post
Demetrius, On Style, 1-2
“Just as poetry is separated by meters—such as half-lines, hexameters, and the rest—so too will sections called clauses [kôla] separate and define prose composition. They allow rests to the speaker and what is spoken and they give the composition boundaries in many places, since it would be long and endless and would just exhaust anyone reading it otherwise.
These clauses are really meant to bring an end to a thought. Sometimes they convey a complete thought on their own, as when Hekataios says at the beginning of his History, “Hekataios speaks thus”. In this a case a whole thought coincides with a single clause and both end together. At another time, a clause doesn’t effect a complete thought, but merely part of one.
For, just as the hand is a whole thing but has individual parts of the whole, such as the fingers and the wrist—each of which has its own particular shape and recognizable parts—so too will the parts of a larger thought which is complete and whole be subsumed within it even though they too are recognizable and defined.”
(1) Ὥσπερ ἡ ποίησις διαιρεῖται τοῖς μέτροις, οἷον ἡμιμέτροις ἢ ἑξαμέτροις ἢ τοῖς ἄλλοις, οὕτω καὶ τὴν ἑρμηνείαν τὴν λογικὴν διαιρεῖ καὶ διακρίνει τὰ καλούμενα κῶλα, καθάπερ ἀναπαύοντα τὸν λέγοντά τε καὶ τὰ λεγόμενα αὐτά, καὶ ἐν πολλοῖς ὅροις ὁρίζοντα τὸν λόγον, ἐπεί τοι μακρὸς ἂν εἴη καὶ ἄπειρος καὶ ἀτεχνῶς πνίγων τὸν λέγοντα.
(2) βούλεται μέντοι διάνοιαν ἀπαρτίζειν τὰ κῶλα ταῦτα, ποτὲ μὲν ὅλην διάνοιαν, οἷον ὡς Ἑκαταῖός φησιν ἐν τῇ ἀρχῇ τῆς ἱστορίας, “Ἑκαταῖος Μιλήσιος ὧδε μυθεῖται”· συνείληπται γὰρ διάνοια τῷ κώλῳ ὅλῳ ὅλη, καὶ ἄμφω συγκαταλήγουσιν. ἐνίοτε μέντοι τὸ κῶλον ὅλην μὲν οὐ συμπεραιοῖ διάνοιαν, μέρος δὲ ὅλης ὅλον. ὡς γὰρ τῆς χειρὸς οὔσης ὅλου τινὸς μέρη αὐτῆς ὅλα ὅλης ἐστίν, οἷον δάκτυλοι καὶ πῆχυς (ἰδίαν γὰρ περιγραφὴν ἔχει τούτων τῶν μερῶν ἕκαστον, καὶ ἴδια μέρη), οὕτω καὶ διανοίας τινὸς ὅλης οὔσης μεγάλης ἐμπεριλαμβάνοιτ᾿ ἂν μέρη τινὰ αὐτῆς ὁλόκληρα ὄντα καὶ αὐτά·