See a law of Constantine himself.
A praefectis autem praetorio provocare, non sinimus. Cod. Justinian. l. vii. tit. lxii. Ieg. 19.
Charisius, a lawyer of the time of Constantine (Heinec. Hist. Juris Romani, p. 349), who admits this law as a fundamental principle of jurisprudence, compares the Praetorian praefects to the masters of the horse of the ancient dictators. Pandect. l. i. tit. xi.