The legions, as they are described by Polybius, in the time of the Punic wars, differed very materially from those which achieved the victories of Caesar, or defended the monarchy of Hadrian and the Antonines.

The constitution of the Imperial legion may be described in a few words.

The heavy-armed infantry, which composed its principal strength, was divided into ten cohorts, and fifty-five companies, under the orders of a correspondent number of tribunes and centurions.

The first cohort, which always claimed the post of honor and the custody of the eagle, was formed of eleven hundred and five soldiers, the most approved for valor and fidelity.

The remaining nine cohorts consisted each of five hundred and fifty-five; and the whole body of legionary infantry amounted to six thousand one hundred men.
Their arms were uniform, and admirably adapted to the nature of their service: an open helmet, with a lofty crest; a breastplate, or coat of mail; greaves on their legs, and an ample buckler on their left arm.